CI: Partially sync workflows and actions with 3.x branch

Stick to `ubuntu:20.04` runners for now, as porting to newer ones implies
fixing a number of compilation warnings which may not be worth the trouble
for an EOL branch.

.github/actions/upload-artifact/action.yml

@@ -0,0 +1,19 @@
+name: Upload Godot artifact
+description: Upload the Godot artifact.
+inputs:
+  name:
+    description: The artifact name.
+    default: "${{ github.job }}"
+  path:
+    description: The path to upload.
+    required: true
+    default: "bin/*"
+runs:
+  using: "composite"
+  steps:
+    - name: Upload Godot Artifact
+      uses: actions/upload-artifact@v4
+      with:
+        name: ${{ inputs.name }}
+        path: ${{ inputs.path }}
+        retention-days: 14