Graduate Ruler API from experimental

The Ruler API has been stable for a long time. Remove it from the
experimental features list in v1-guarantees.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Friedrich Gonzalez <1517449+friedrichg@users.noreply.github.com>

Author: friedrichg

docs/configuration/v1-guarantees.md

@@ -41,7 +41,6 @@ Currently experimental features are:
 - S3 Server Side Encryption (SSE) using KMS (including per-tenant KMS config overrides).
 - Azure blob storage.
 - Zone awareness based replication.
-- Ruler API (to PUT rules).
 - Alertmanager:
   - API (enabled via `-experimental.alertmanager.enable-api`)
   - Sharding of tenants across multiple instances (enabled via `-alertmanager.sharding-enabled`)