feat: replace manual pruning with buildkit native gc

Remove max-cache-size-mb input and manual buildctl prune from the post
phase. Enable BuildKit's native garbage collection with a time-based
policy (keepDuration=192h / 8 days). Layers unused for longer than the
retention period are automatically cleaned up; active layers survive
indefinitely regardless of total cache size.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: piob-io

action.yml

@@ -39,9 +39,6 @@ inputs:
   max-parallelism:
     description: "Maximum number of concurrent BuildKit RUN steps. Defaults to the number of vCPUs on the runner."
     required: false
-  max-cache-size-mb:
-    description: "Amount of build cache to retain after pruning, in MB (e.g., 409600 for 400GB). If not set, pruning is skipped."
-    required: false
 runs:
   using: node24
   main: dist/index.js