CI: Update first-party GitHub Actions from v3 to v4

GitHub Actions has deprecated Actions based on node16. The v4 actions
are based on node20. Replace first-party v3 actions with their v4
counterparts.

See:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

.github/actions/flatpak-builder-lint/action.yaml

@@ -33,7 +33,7 @@ runs:
             ;;
         esac
 
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         repository: flathub/flatpak-builder-lint
         ref: v2.0.13

.github/actions/generate-docs/action.yaml

@@ -54,7 +54,7 @@ runs:
         target_path: '../home/_build'
         pre_build_commands: 'pip install -Iv sphinx==5.1.1'
 
-    - uses: actions/upload-artifact@v3
+    - uses: actions/upload-artifact@v4
       with:
         name: ${{ steps.setup.outputs.artifactName }} ${{ steps.setup.outputs.commitHash }}
         path: |

.github/actions/services-validator/action.yaml

@@ -82,7 +82,7 @@ runs:
 
     - name: Restore Timestamp Cache ⏳
       if: fromJSON(inputs.runServiceChecks)
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: ${{ github.workspace }}/other
         key: service-check-${{ github.run_id }}
@@ -103,7 +103,7 @@ runs:
         : Check for defunct services 📉
         python3 -u .github/scripts/utils.py/check-services.py
 
-    - uses: actions/upload-artifact@v3
+    - uses: actions/upload-artifact@v4
       if: fromJSON(inputs.runServiceChecks)
       with:
         name: timestamps

.github/actions/steam-upload/action.yaml

@@ -70,7 +70,7 @@ runs:
 
     - name: Download Nightly Assets 🌙
       id: asset-info-nightly
-      uses: actions/download-artifact@v3
+      uses: actions/download-artifact@v4
       if: github.event_name == 'schedule'
 
     - name: Download Assets 📥
@@ -285,7 +285,7 @@ runs:
         popd
 
     - name: Upload Steam build logs
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       with:
         name: steam-build-logs
         path: ${{ github.workspace }}/steam/build/*.log