Unify the Godot 4.7.2 CI environment and cache it

Update project features and CI installs/templates to Godot 4.7.2, remove the
old version matrix, and cache the Godot binary and export templates with Gitea
Actions cache. Update release/docs/version references accordingly. Project
import and headless tests pass locally under 4.7.2.

tools/build_releases.sh

@@ -2,7 +2,7 @@
 # Build distributable desktop executables with the Godot export pipeline.
 #
 # Requirements:
-#   - Godot 4.3 (or the version in project.godot `config/features`) on PATH,
+#   - Godot 4.7.2 (or the version in project.godot `config/features`) on PATH,
 #     with export templates installed (see docs/RELEASING.md).
 #   - Run from the repository root.
 #