Bump chainguard-dev/cosign from 0.0.26 to 0.0.28

Bumps [chainguard-dev/cosign](https://github.com/chainguard-dev/terraform-provider-cosign) from 0.0.26 to 0.0.28.
- [Release notes](https://github.com/chainguard-dev/terraform-provider-cosign/releases)
- [Changelog](https://github.com/chainguard-dev/terraform-provider-cosign/blob/main/.goreleaser.yml)
- [Commits](chainguard-dev/terraform-provider-cosign@v0.0.26...v0.0.28)

---
updated-dependencies:
- dependency-name: chainguard-dev/cosign
  dependency-version: 0.0.28
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

main.tf

@@ -2,7 +2,7 @@ terraform {
   required_providers {
     cosign = {
       source  = "chainguard-dev/cosign"
-      version = "0.0.26"
+      version = "0.0.28"
     }
     apko = {
       source  = "chainguard-dev/apko"