ci: pin third-party GitHub Actions to commit SHAs (#5972)

Replaces every floating tag in our workflow and composite action files
with an immutable 40-character commit SHA, keeping the original `# vX`
comment so Dependabot can still propose version bumps. 186 occurrences
across 25 workflows and 2 composite actions.

Also widens the github-actions Dependabot entry to use the plural
`directories` key with `/.github/actions/*` so composite actions under
`.github/actions/<name>/action.yml` are kept up to date. Previously
Dependabot only scanned `.github/workflows` and the repo-root
`action.yml`, leaving our `python-setup` and `sample-validation-setup`
composite actions unmaintained.

.github/workflows/markdown-link-check.yml

@@ -19,13 +19,13 @@ jobs:
     runs-on: ubuntu-22.04
     # check out the latest version of the code
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           persist-credentials: false
 
       # Checks the status of hyperlinks in all files
       - name: Run linkspector
-        uses: umbrelladocs/action-linkspector@v1
+        uses: umbrelladocs/action-linkspector@963b6264d7de32c904942a70b488d3407453049e # v1
         with:
           reporter: local
           filter_mode: nofilter