Pin our GitHub Actions dependencies (#18255)

After the [recent supply chain attack](https://www.wiz.io/blog/new-github-action-supply-chain-attack-reviewdog-action-setup)
in `tj-actions/changed-files` and actions based on it, it's become clear
that relying on git tags to pin our dependencies is not enough (as tags
can simply be replaced). Therefore we need to switch to hashes.

Dependabot should continue to update these dependencies for us.

Best reviewed commit-by-commit. Though if CI passes, we're *probably*
fine.

.github/workflows/docs.yaml

@@ -50,7 +50,7 @@ jobs:
     needs:
       - pre
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           # Fetch all history so that the schema_versions script works.
           fetch-depth: 0
@@ -64,7 +64,7 @@ jobs:
         run: echo 'window.SYNAPSE_VERSION = "${{ needs.pre.outputs.branch-version }}";' > ./docs/website_files/version.js
 
       - name: Setup python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@42375524e23c412d93fb67b49958b491fce71c38 # v5.4.0
         with:
           python-version: "3.x"