Remove legacy images

Author: ubruhin

README.md

@@ -12,20 +12,6 @@ The built images are hosted at
 
 ## Available Tags
 
-### `ubuntu-18.04-qt5.15.2`
-
-Based on Ubuntu 18.04, containing Qt 5.15.2 from
-[download.qt.io](https://download.qt.io) and OpenCascade OCCT from
-[github/Open-Cascade-SAS/OCCT](https://github.com/Open-Cascade-SAS/OCCT).
-This image is intended for deployment of official binary releases of LibrePCB,
-which should be linked against an old version of `glibc` (for maximum
-compatibility) but still using a recent Qt version (to get the latest
-features/bugfixes of Qt).
-
-In addition, this image contains
-[`linuxdeployqt`](https://github.com/probonopd/linuxdeployqt) to build the
-AppImage.
-
 ### `ubuntu-20.04-qt6.6`
 
 Based on Ubuntu 20.04, containing Qt 6.6.x from
@@ -40,9 +26,6 @@ In addition, this image contains
 [`linuxdeployqt`](https://github.com/probonopd/linuxdeployqt) to build the
 AppImage.
 
-This image will deprecate `ubuntu-18.04-qt5.15.2` once we definitively
-switch from Qt5 to Qt6. Currently we provide both Qt5 and Qt6 builds.
-
 ### `ubuntu-20.04`
 
 Based on Ubuntu 20.04, containing Qt from the official Ubuntu package
@@ -59,12 +42,6 @@ Based on Ubuntu 22.04, containing Qt and OpenCascade OCE from the official
 Ubuntu package repository. This image is intended to check if LibrePCB
 compiles on a standard Ubuntu 22.04.
 
-### `windowsservercore-ltsc2019-qt5.15.2-32bit`
-
-Based on Windows Server Core LTSC2019 with Qt5.15.2, MinGW 8.1 32-bit
-and OpenCascade OCCT 7.7.2. This image is intended for deployment of official
-32-bit binary releases of LibrePCB for Windows.
-
 ### `windowsservercore-ltsc2019-qt6.6-64bit`
 
 Based on Windows Server Core LTSC2019 with Qt6.6.x, MinGW 11.2.0 64-bit