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/dotnet-format.yml

@@ -30,7 +30,7 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v6
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           fetch-depth: 0
           persist-credentials: false
@@ -42,7 +42,7 @@ jobs:
       - name: Get changed files
         id: changed-files
         if: github.event_name == 'pull_request'
-        uses: jitterbit/get-changed-files@v1
+        uses: jitterbit/get-changed-files@b17fbb00bdc0c0f63fcf166580804b4d2cdc2a42 # v1
         continue-on-error: true
 
       - name: No C# files changed