Docs: Updated alert box style

Related to: https://docs.gitlab.com/ee/development/documentation/styleguide/index.html\#alert-boxes

doc/administration/packages/container_registry.md

@@ -1139,7 +1139,7 @@ and a simple solution would be to enable relative URLs in the Registry.
 
 You can use the Container Registry debug server to diagnose problems. The debug endpoint can monitor metrics and health, as well as do profiling.
 
-CAUTION: **Warning:**
+WARNING:
 Sensitive information may be available from the debug endpoint.
 Access to the debug endpoint must be locked down in a production environment.
 

doc/topics/autodevops/customize.md

@@ -615,7 +615,7 @@ With `INCREMENTAL_ROLLOUT_MODE` set to `manual` and with `STAGING_ENABLED`
 
 ![Rollout and staging enabled](img/rollout_staging_enabled.png)
 
-WARNING: **Deprecation:**
+WARNING:
 Before GitLab 11.4, the presence of the `INCREMENTAL_ROLLOUT_ENABLED` environment
 variable enabled this feature. This configuration is deprecated, and is scheduled to be
 removed in the future.