Use Gradle 8.11 as the minimum version for cache-cleanup

The cache-cleanup API has changed, so the init-script that worked with
Gradle 8.9 no longer works with 8.11.
We now provision and use Gradle 8.11 for cache cleanup.

This provides a band-aid fix for #417 but that issue will still impact
any build configured to run with Gradle > 8.11

sources/.tool-versions

@@ -1,3 +1,3 @@
 # Configuration file for asdf version manager
 nodejs 20.10.0
-gradle 8.10.2
+gradle 8.11