chore(rds): add MariaDB engine version 11.8.5 (#36279)

Ref:https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/MariaDB.Concepts.VersionMgmt.html

### Description of how you validated changes

```
% aws rds describe-db-engine-versions --engine mariadb --query "DBEngineVersions[?EngineVersion=='11.8.5'].[DBEngineVersionDescription,EngineVersion,DBParameterGroupFamily,MajorEngineVersion,Status]"

- - MariaDB 11.8.１
  - 11.8.5
  - mariadb11.8
  - '11.8'
  - available 
  ```

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: matoom-nomu

packages/aws-cdk-lib/aws-rds/lib/instance-engine.ts

@@ -664,6 +664,8 @@ export class MariaDbEngineVersion {
 
   /** Version "11.8.3". */
   public static readonly VER_11_8_3 = MariaDbEngineVersion.of('11.8.3', '11.8');
+  /** Version "11.8.5". */
+  public static readonly VER_11_8_5 = MariaDbEngineVersion.of('11.8.5', '11.8');
 
   /**
    * Create a new MariaDbEngineVersion with an arbitrary version.

packages/aws-cdk-lib/aws-rds/test/instance-engine.test.ts

@@ -312,6 +312,7 @@ describe('instance engine', () => {
       ['11.4.7', rds.MariaDbEngineVersion.VER_11_4_7],
       ['11.4.8', rds.MariaDbEngineVersion.VER_11_4_8],
       ['11.8.3', rds.MariaDbEngineVersion.VER_11_8_3],
+      ['11.8.5', rds.MariaDbEngineVersion.VER_11_8_5],
     ])('is passed correctly for %s', (engineVersion, version) => {
       // WHEN
       const stack = new cdk.Stack();