[Cherry pick] Pin GitHub Actions to full-length commit SHAs (#704)

Author: Copilot

.github/actions/spelling/allow.txt

@@ -24,6 +24,7 @@ cla
 codeowner
 codeowners
 coenraads
+cooldown
 dbaeumer
 devdocs
 eamodio

.github/dependabot.yml

@@ -0,0 +1,11 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    groups:
+      github-actions:
+        patterns: ["*"]
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7

.github/workflows/spellCheck.yml

@@ -91,7 +91,7 @@ jobs:
     steps:
       - name: check-spelling
         id: spelling
-        uses: check-spelling/check-spelling@v0.0.26
+        uses: check-spelling/check-spelling@cfb6f7e75bbfc89c71eaa30366d0c166f1bd9c8c # v0.0.26
         with:
           suppress_push_for_open_pull_request: ${{ github.actor != 'dependabot[bot]' && 1 }}
           checkout: true
@@ -140,7 +140,7 @@ jobs:
     if: (success() || failure()) && needs.spelling.outputs.followup && contains(github.event_name, 'pull_request')
     steps:
       - name: comment
-        uses: check-spelling/check-spelling@v0.0.26
+        uses: check-spelling/check-spelling@cfb6f7e75bbfc89c71eaa30366d0c166f1bd9c8c # v0.0.26
         with:
           checkout: true
           task: ${{ needs.spelling.outputs.followup }}