CI: Make extension API compatibility check mandatory

This means that any PR which breaks the extension API should
handle it properly, that is:

- Add compatibility methods to ensure that existing function hashes work
- Document the changes in the relevant misc/extension_api_validation/ file

(cherry picked from commit a890457693)

.github/workflows/linux_builds.yml

@@ -165,7 +165,7 @@ jobs:
       - name: Check for GDExtension compatibility
         if: ${{ matrix.api-compat }}
         run: |
-          ./misc/scripts/validate_extension_api.sh "${{ matrix.bin }}" || true # don't fail the CI for now
+          ./misc/scripts/validate_extension_api.sh "${{ matrix.bin }}"
 
       # Download and run the test project
       - name: Test Godot project