Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/add-milestone.yml

@@ -8,14 +8,14 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Actions
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           repository: "karrtikr/vscode-github-triage-actions"
           ref: stable
           path: ./actions
       - name: Checkout Repo
         if: github.event_name != 'issues'
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           path: ./repo
           fetch-depth: 0