chore(apprunner-alpha): add runtimes PYTHON_311 and NODEJS_18 (#30149)

### Issue # (if applicable)

Closes #30146.

### Reason for this change

Runtimes Python 3.11 and NodeJS 18 were missing.

### Description of changes

Runtimes Python 3.11 and NodeJS 18 were added.

### Description of how you validated changes

No tests were added, as it seems not required for this type of change.

### Checklist
- [*] 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: mellevanderlinde

packages/@aws-cdk/aws-apprunner-alpha/lib/service.ts

@@ -202,6 +202,11 @@ export class Runtime {
    */
   public static readonly NODEJS_16 = Runtime.of('NODEJS_16')
 
+  /**
+   * NodeJS 18
+   */
+  public static readonly NODEJS_18 = Runtime.of('NODEJS_18')
+
   /**
    * PHP 8.1
    */
@@ -212,6 +217,11 @@ export class Runtime {
    */
   public static readonly PYTHON_3 = Runtime.of('PYTHON_3')
 
+  /**
+   * Python 3.11
+   */
+  public static readonly PYTHON_311 = Runtime.of('PYTHON_311')
+
   /**
    * Ruby 3.1
    */