CI: Bump actions/checkout from 2 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/stable-compilation.yml

@@ -21,7 +21,7 @@ jobs:
       image: ghcr.io/flathub-infra/flatpak-github-actions:kde-6.10
       options: --privileged
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v6
       with:
         bundle: EasyRPG-Editor.flatpak
@@ -71,10 +71,10 @@ jobs:
           git clone https://github.com/Microsoft/vcpkg.git ../vcpkg
 
       - name: Clone Repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v6
         if: github.event.inputs.git-ref == ''
       - name: Clone Repository (with custom ref)
-        uses: actions/checkout@v2
+        uses: actions/checkout@v6
         if: github.event.inputs.git-ref != ''
         with:
           ref: ${{ github.event.inputs.git-ref }}