chore(rds): add MySQL 5.7.40 (#22917)

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Add missing Database engine versions.

ref: https://aws.amazon.com/about-aws/whats-new/2022/11/amazon-rds-mysql-supports-new-minor-version-5-7-40/

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### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: neilkuan

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

@@ -558,6 +558,8 @@ export class MysqlEngineVersion {
   public static readonly VER_5_7_38 = MysqlEngineVersion.of('5.7.38', '5.7');
   /** Version "5.7.39". */
   public static readonly VER_5_7_39 = MysqlEngineVersion.of('5.7.39', '5.7');
+  /** Version "5.7.40". */
+  public static readonly VER_5_7_40 = MysqlEngineVersion.of('5.7.40', '5.7');
 
   /** Version "8.0" (only a major version, without a specific minor version). */
   public static readonly VER_8_0 = MysqlEngineVersion.of('8.0', '8.0');