chore: pin third-party GitHub Actions to SHAs + enable Dependabot

.github/dependabot.yml

@@ -0,0 +1,22 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 10
+    groups:
+      actions-minor-patch:
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"
+      actions-major:
+        patterns:
+          - "*"
+        update-types:
+          - "major"
+    cooldown:
+      default-days: 7

.github/workflows/gardening.yml

@@ -31,7 +31,7 @@ jobs:
           node-version: lts/*
 
      - name: Wait for prior instances of the workflow to finish
-       uses: softprops/turnstyle@v1
+       uses: softprops/turnstyle@8db075d65b19bf94e6e8687b504db69938dc3c65 # v0.1.5
        env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}