SCORU/Node/Store: expose GC function for all stores

Context

This MR exposes a gc function in the module Store, which removes everything below a given level.

The gc function is implemented only for the last version of the store as it will be migrated on startup.

Depends on !10050 (merged) and !10236 (merged).

Manually testing the MR

dune exec src/lib_smart_rollup_node/test/main.exe -- -f test_store_gc.ml