Use SHAs for GHA versions

Per GitHub's security guidelines, GHAs should be pinned using full
length commit SHAs instead of tags.

The SHAs are of the commits currently resolved by the versions.

Even "trusted" GHAs from GitHub developers are pinned because it's
possible their repo rights could be compromised and a malicious GHA
published. These core repos are not frequently substantially updated.

Submariner-internal GHAs are left pinned at devel because we want
automatic updates from Shipyard's shared tooling.

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

.github/workflows/periodic.yml

@@ -12,16 +12,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out the repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f
 
       - name: Run markdown-link-check
-        uses: gaurav-nelson/github-action-markdown-link-check@v1
+        uses: gaurav-nelson/github-action-markdown-link-check@9710f0fec812ce0a3b98bef4c9d842fc1f39d976
         with:
           config-file: ".markdownlinkcheck.json"
 
       - name: Raise an Issue to report broken links
         if: ${{ failure() }}
-        uses: peter-evans/create-issue-from-file@v2.3.2
+        uses: peter-evans/create-issue-from-file@a04ce672e3acedb1f8e416b46716ddfd09905326
         with:
           title: Broken link detected by CI
           content-filepath: .github/ISSUE_TEMPLATE/broken-link.md