Use FFmpeg 6 only for reference resource CI job (#1198)

Author: NicolasHug

.github/workflows/reference_resources.yaml

@@ -53,7 +53,10 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ['3.10']
-        ffmpeg-version-for-tests: ['4.4.2', '5.1.2', '6.1.1', '7.0.1']
+        # Traditionally we generate the resources locally on FFmpeg 4 or 6.
+        # The exact version shouln't matter as long as the unit tests pass
+        # across all version for a given generated resource.
+        ffmpeg-version-for-tests: ['6.1.1']
     steps:
       - name: Check out repo
         uses: actions/checkout@v3