db: commitlog: don't print INFO logs on shutdown

The intention was for these logs to be printed during the
database shutdown sequence, but it was overlooked that it's not
the only place where commitlog::shutdown is called.
Commitlogs are started and shut down periodically by hinted handoff.
When that happens, these messages spam the log.

Fix that by adding INFO commitlog shutdown logs to database::stop,
and change the level of the commitlog::shutdown log call to DEBUG.

Fixes #11508

Closes #11536

db/commitlog/commitlog.cc

@@ -2052,7 +2052,7 @@ future<> db::commitlog::segment_manager::shutdown() {
         }
     }
     co_await _shutdown_promise->get_shared_future();
-    clogger.info("Commitlog shutdown complete");
+    clogger.debug("Commitlog shutdown complete");
 }
 
 void db::commitlog::segment_manager::add_file_to_dispose(named_file f, dispose_mode mode) {