CI: Fix CodeQL definition

It lacked an appropriate setting for the `category` parameter. When
introducing a matrix axis (here: sqla-version), you will have to teach
code scanning about it.

This has been discovered by the CodeQL team. Thanks a stack!

  github/codeql-action#1411 (comment)

.github/workflows/codeql.yml

@@ -54,4 +54,4 @@ jobs:
       - name: Perform CodeQL Analysis
         uses: github/codeql-action/analyze@v2
         with:
-          category: "/language:${{ matrix.language }}"
+          category: "/language:${{ matrix.language }}/sqla-version:${{ matrix.sqla-version }}"