feat(neptune-alpha): add engine version v1.5.0.0

[vaibhavm99]

Issue # (if applicable)

Closes #XXXXX

Reason for this change

Neptune engine version 1.5.0.0 not available in the CDK construct library.

Description of changes

Added Neptune engine version 1.5.0.0 and parameter group family neptune1.5.

Description of how you validated changes

Integration test (integ.cluster-ev15.ts)

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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[leonmk-aws]

Thanks for the PR! Unfortunately, Neptune engine version 1.5.0.0 and the neptune1.5 parameter group family don't exist yet. They are not mentioned in the Neptune engine releases documentation (https://docs.aws.amazon.com/neptune/latest/userguide/engine-releases.html), and deploying this returns:

ParameterGroupFamily neptune1.5 is not a valid parameter group family (Service: Neptune, Status Code: 400)

Closing for now — please re-open this PR once the version becomes available.

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