Bump actions/checkout from 1 to 4 (#351)

Bumps [actions/checkout](https://github.com/actions/checkout) from 1 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v1...v4)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/gradle-build.yml

@@ -5,7 +5,7 @@ jobs:
     name: "Building"
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: gradle/wrapper-validation-action@v1
     - uses: gradle/gradle-build-action@v2
       name: Check Fladle

.github/workflows/mkdocs-deploy.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout master
-        uses: actions/checkout@v1
+        uses: actions/checkout@v4
 
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@1.12