fix(mito): make do_region_gc pub(crate) to fix private_interfaces warning

clippy runs with -D warnings and this private_interfaces warning fails the
backport CI on release/v1.2.

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

src/mito2/src/gc.rs

@@ -410,7 +410,7 @@ impl LocalGcWorker {
             region_present = region.is_some()
         )
     )]
-    pub async fn do_region_gc(
+    pub(crate) async fn do_region_gc(
         &self,
         region_id: RegionId,
         region: Option<MitoRegionRef>,