feat(codebuild): add support for macOS 15 runners (#35836)

### Reason for this change

Although CodeBuild supports the macOS base:15 image ([ref](https://docs.aws.amazon.com/codebuild/latest/userguide/available-runtimes.html#:~:text=arm%2Dbase%3A14-,macos%2Darm%2Dbase%3A15,-clang)), the CDK does not support this. 

### Description of changes

I added an option for the BASE_15 image. This simply involved copying the logic for BASE_14 and changing 14 to 15.

### Describe any new or updated permissions being added

NA.

### Description of how you validated changes

I added unit and integration tests for the base:15 image following the example of the existing tests for the base:14 image. 

Unit tests (aws-codebuild/test/project.test.ts) passed locally:
```
    ✓ has build image for Base 14 (32 ms)
    ✓ has build image for Base 15 (12 ms)
    ✓ can set macOS fleet with BASE_14 (11 ms)
    ✓ can set macOS fleet with BASE_15 (10 ms)
    ✓ can set imported macOS fleet with BASE_14 (11 ms)
    ✓ can set imported macOS fleet with BASE_15 (11 ms)
```
The integration tests (aws-codebuild/test/integ.project-macos-fleet-base{14/15}.js) were not as straightforward. In the CodeBuild stack logs in my console, the stacks were able to be created successfully but failed to delete. I believe this is expected with macOS fleets and for these test cases, especially as the base:14 test already existed in the codebase. I have included snapshots, as required.

### 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)

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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: ayush-panta