Khepri: Mark khepri_db as stable

[Why]
The intent is to have it stable and enabled by default for new
deployment in RabbitMQ 4.1.x.

To prepare for this goal, it is time to mark the feature flag as stable
to let us iron out the library and its integration into RabbitMQ.

This is not a commitment at this stage: we will revisit this near the
beginning of the release cycle and commit to it or revert to
experimental.

deps/rabbit/src/rabbit_core_ff.erl

@@ -148,8 +148,7 @@
    {khepri_db,
     #{desc          => "New Raft-based metadata store.",
       doc_url       => "https://www.rabbitmq.com/docs/next/metadata-store",
-      stability     => experimental,
-      experiment_level => supported,
+      stability     => stable,
       depends_on    => [feature_flags_v2,
                         direct_exchange_routing_v2,
                         maintenance_mode_status,