Add GCC 15 CI build

Author: offa

.github/workflows/ci.yml

@@ -8,8 +8,8 @@ jobs:
     strategy:
       matrix:
         compiler:
+          - gcc:15
           - gcc:14
-          - gcc:13
           - clang:20
           - clang:19
     container:

.github/workflows/codeql.yml

@@ -10,7 +10,7 @@ jobs:
   codeql:
     runs-on: ubuntu-latest
     container:
-      image: "registry.gitlab.com/offa/docker-images/gcc:14"
+      image: "registry.gitlab.com/offa/docker-images/gcc:15"
     name: "CodeQL"
     steps:
       - name: Checkout