Fix xlint findings

[chcg]

What feature do you want to see added?

Current xlint findings:

[INFO] Compiling 26 source files to \JiraTestResultReporter-plugin\target\classes

• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraUtils.java:[93,24] getInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestDataPublisher.java:[427,57] getInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestDataPublisher.java:[801,47] unchecked method invocation: method newInstancesFromHeteroList in class hudson.model.Descriptor is applied to given types
  required: org.kohsuke.stapler.StaplerRequest,java.lang.Object,java.util.Collection<? extends hudson.model.Descriptor>
  found: org.kohsuke.stapler.StaplerRequest,java.lang.Object,java.util.List
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestDataPublisher.java:[801,105] unchecked conversion
  required: java.util.Collection<? extends hudson.model.Descriptor>
  found: java.util.List
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestDataPublisher.java:[801,47] unchecked conversion
  required: java.util.List<org.jenkinsci.plugins.JiraTestResultReporter.config.AbstractFields>
  found: java.util.List
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestDataPublisher.java:[843,27] getInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JobConfigMapping.java:[265,35] getInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/config/SelectableArrayFields.java:[162,61] unchecked cast
  required: com.atlassian.jira.rest.client.api.IdentifiableEntity<java.lang.Long>
  found: java.lang.Object
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestResultReporterPlugin.java:[12,8] Plugin() in hudson.Plugin has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/config/SelectableFields.java:[149,61] unchecked cast
  required: com.atlassian.jira.rest.client.api.IdentifiableEntity<java.lang.Long>
  found: java.lang.Object
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/api/TestToIssueMappingApi.java:[47,32] getActiveInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/api/TestToIssueMappingApi.java:[56,40] getActiveInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/api/TestToIssueMappingApi.java:[65,36] getActiveInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestData.java:[54,53] hudson.tasks.junit.TestObject in hudson.tasks.junit has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestData.java:[54,53] hudson.tasks.junit.TestObject in hudson.tasks.junit has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestData.java:[54,53] hudson.tasks.junit.TestObject in hudson.tasks.junit has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/TestToIssueMapping.java:[60,31] getInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/TestToIssueMapping.java:[103,90] unchecked cast
  required: java.util.HashMap<java.lang.String,java.lang.String>
  found: java.lang.Object
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestAction.java:[96,32] getInstance() in jenkins.model.Jenkins has been deprecated
• [WARNING] //JiraTestResultReporter-plugin/src/main/java/org/jenkinsci/plugins/JiraTestResultReporter/JiraTestAction.java:[242,50] getInstance() in jenkins.model.Jenkins has been deprecated

Upstream changes

No response

Are you interested in contributing this feature?

No response

[chcg]

Xlint for maven pom.xml:

+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.8.0</version>
+        <configuration>
+          <compilerArgs>
+            <arg>-Xlint:deprecation</arg>
+            <arg>-Xlint:unchecked</arg>
+          </compilerArgs>
+        </configuration>
+      </plugin>