meson CI: test oldest and newest compilers only

No need for the ones in the middle.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

.github/workflows/on_PR_meson.yaml

@@ -12,7 +12,7 @@ jobs:
     name: Linux-GCC${{matrix.cxx}}-${{matrix.deps}}
     strategy:
       matrix:
-        cxx: ['7', '8', '9', '10']
+        cxx: ['7', '10']
         deps: ['forcefallback', 'default']
     steps:
       - uses: actions/checkout@v3
@@ -34,7 +34,7 @@ jobs:
     name: Linux-Clang${{matrix.cxx}}-${{matrix.deps}}
     strategy:
       matrix:
-        cxx: ['7', '8', '9', '10', '12']
+        cxx: ['7', '12']
         deps: ['forcefallback', 'default']
     steps:
       - uses: actions/checkout@v3