Retire gh_ci/yaml_snippets submodule; sparse-fetch from mirrors_service

Drop the gh_ci/yaml_snippets submodule in favor of a second checkout
step that sparse-fetches src/backend/yaml_snippets from the
mirrors_service branch and moves it into gh_ci/yaml_snippets. Schema
paths and imports in gh_ci/config_checker.py keep working unchanged.

Side benefits:
- master CI now validates mirror configs against the exact parser the
  service runs at runtime, so schema drift between branches is no
  longer possible.
- actions/checkout and actions/setup-python bumped from v2 (EOL) to v4
  while the workflow was being edited.

Depends on the corresponding change on the mirrors_service branch which
moves yaml_snippets from a submodule to regular files at the same path.

Author: jonathanspw

.github/workflows/ci.yml

@@ -23,11 +23,20 @@ jobs:
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
+
+      - name: Fetch yaml_snippets from mirrors_service branch
+        uses: actions/checkout@v4
         with:
-          submodules: recursive
+          ref: mirrors_service
+          path: _mirrors_service
+          sparse-checkout: src/backend/yaml_snippets
+          sparse-checkout-cone-mode: false
+
+      - name: Stage yaml_snippets for config_checker
+        run: mv _mirrors_service/src/backend/yaml_snippets gh_ci/yaml_snippets
 
-      - uses: actions/setup-python@v2
+      - uses: actions/setup-python@v4
         with:
           python-version: 3.9