Change: [CI] always use "latest" image for our runners

.github/workflows/ci-build.yml

@@ -99,7 +99,7 @@ jobs:
 
     name: Linux (${{ matrix.name }})
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     env:
       CC: ${{ matrix.compiler }}
       CXX: ${{ matrix.cxxcompiler }}

.github/workflows/release-linux.yml

@@ -12,7 +12,7 @@ jobs:
   linux:
     name: Linux (Generic)
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     container:
       # manylinux2014 is based on CentOS 7, but already has a lot of things
       # installed and preconfigured. It makes it easier to build OpenTTD.

.github/workflows/release-macos.yml

@@ -12,7 +12,7 @@ jobs:
   macos:
     name: MacOS
 
-    runs-on: macos-12
+    runs-on: macos-latest
     env:
       MACOSX_DEPLOYMENT_TARGET: 10.13