CI: linux workflows to run under ubuntu-latest.

Author: sezero

.github/workflows/main.yml

@@ -18,8 +18,8 @@ jobs:
         - { name: Windows (MSVC+CMake),         os: windows-latest, shell: sh,   cmake: '-DPerl_ROOT=C:/Strawberry/perl/bin/ -DSDLTTF_VENDORED=ON -GNinja', msvc: 1, shared: 1, static: 0, artifact: 'SDL3_ttf-VC-x64' }
         - { name: Windows (mingw64+CMake),      os: windows-latest, shell: 'msys2 {0}', msystem: mingw64, msys-env: mingw-w64-x86_64, shared: 1, static: 0,
             cmake: '-DSDLTTF_VENDORED=OFF -G "Ninja Multi-Config"', artifact: 'SDL3_ttf-mingw64' }
-        - { name: Linux,                        os: ubuntu-20.04,   shell: sh,   cmake: '-DSDLTTF_VENDORED=ON -GNinja', shared: 1, static: 0, artifact: 'SDL3_image-linux-x64' }
-        - { name: 'Linux (static)',             os: ubuntu-20.04,   shell: sh,   cmake: '-GNinja -DBUILD_SHARED_LIBS=OFF', shared: 0, static: 1, artifact: 'SDL3_Image-static-linux-x64' }
+        - { name: Linux,                        os: ubuntu-latest,  shell: sh,   cmake: '-DSDLTTF_VENDORED=ON -GNinja', shared: 1, static: 0, artifact: 'SDL3_image-linux-x64' }
+        - { name: 'Linux (static)',             os: ubuntu-latest,  shell: sh,   cmake: '-GNinja -DBUILD_SHARED_LIBS=OFF', shared: 0, static: 1, artifact: 'SDL3_Image-static-linux-x64' }
         - { name: Macos,                        os: macos-latest,   shell: sh,   cmake: '-DSDLTTF_VENDORED=ON -GNinja', shared: 1, static: 0, artifact: 'SDL3_image-macos' }
 
     steps: