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.

project.godot

@@ -14,7 +14,7 @@ config/name="Gig Line"
 config/description="Workplace security simulation about observation, access verification, and proportionate escalation."
 config/version="0.9.0"
 run/main_scene="res://scenes/main.tscn"
-config/features=PackedStringArray("4.3", "GL Compatibility")
+config/features=PackedStringArray("4.7", "GL Compatibility")
 
 [autoload]