ci: add GCC 15,16 coverage

Add CI coverage for GCC 15, 16

Author: msune

.github/workflows/ci.yaml

@@ -78,7 +78,7 @@ jobs:
   ubuntu_24_04_gcc:
     strategy:
       matrix:
-        version: [13, 14]
+        version: [13, 14, 15, 16]
     runs-on: ubuntu-24.04
     env:
       CPP: cpp-${{ matrix.version }}