feat(rds): the unversioned Cluster engine constants are no longer dep…

…recated (#10605)

The static constants in the DatabaseClusterEngine class were all deprecated in favor of using versioned engines.
However, it turns out that it's very common to use the unversioned engines for serverless Clusters,
for which the instances are fully managed, so updates are a lot less risky.
Because of that, un-deprecate the constants, instead adding a note that we don't recommend using them for non-serverless Clusters.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-rds/lib/cluster-engine.ts

@@ -554,23 +554,26 @@ export class DatabaseClusterEngine {
   /**
    * The unversioned 'aurora' cluster engine.
    *
-   * @deprecated using unversioned engines is an availability risk.
+   * **Note**: we do not recommend using unversioned engines for non-serverless Clusters,
+   *   as that can pose an availability risk.
    *   We recommend using versioned engines created using the {@link aurora()} method
    */
   public static readonly AURORA: IClusterEngine = new AuroraClusterEngine();
 
   /**
    * The unversioned 'aurora-msql' cluster engine.
    *
-   * @deprecated using unversioned engines is an availability risk.
+   * **Note**: we do not recommend using unversioned engines for non-serverless Clusters,
+   *   as that can pose an availability risk.
    *   We recommend using versioned engines created using the {@link auroraMysql()} method
    */
   public static readonly AURORA_MYSQL: IClusterEngine = new AuroraMysqlClusterEngine();
 
   /**
    * The unversioned 'aurora-postgresql' cluster engine.
    *
-   * @deprecated using unversioned engines is an availability risk.
+   * **Note**: we do not recommend using unversioned engines for non-serverless Clusters,
+   *   as that can pose an availability risk.
    *   We recommend using versioned engines created using the {@link auroraPostgres()} method
    */
   public static readonly AURORA_POSTGRESQL: IClusterEngine = new AuroraPostgresClusterEngine();