CI: Bump actions/checkout from 2 to 6 and actions/upload-artifact from 4 to 7

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)

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

Author: dependabot[bot]

.github/workflows/stable-compilation.yml

@@ -25,7 +25,7 @@ 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
@@ -157,7 +157,7 @@ jobs:
             --plugin qt
 
       - name: Upload AppImage artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: EasyRPG-Editor-AppImage
           path: ./EasyRPG*.AppImage