eventbridge-cloudwatch-cdk: Update aws-cdk-lib

[kakakakakku]

Issue #, if available:
N/A

Description of changes:
Hi😀 Thanks for the useful patterns!

To keep this pattern maintainable, I updated the aws-cdk-lib.

Check

cdk deploy completed successfully and works good.

$ aws events put-events --entries file://event.json
{
    "FailedEntryCount": 0,
    "Entries": [
        {
            "EventId": "36769164-46ea-44fb-8980-393787ba17e8"
        }
    ]
}

Thank you😀

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[parikhudit]

Thanks for bumping the version!

One observation: aws-cdk-lib==2.258.0 declares requires_python >=3.10 on PyPI, whereas the old 2.13.0 pin supported much older interpreters. The README's Requirements section only says "Python, pip, virtuenv installed" with no minimum version. A user on Python 3.9 or earlier could now get a hard pip install failure after this bump. This is a documentation gap surfaced (not caused) by the upgrade. Appreciate if you could cover that.

[kakakakakku]

@parikhudit
Thanks for the comment. I agree with you. Making the prerequisites explicit is helpful for users of this pattern!

Python 3.9 has already reached EOL, so I've updated the README to state Python 3.10+, matching the requirement of aws-cdk-lib.
https://devguide.python.org/versions/
https://pypi.org/project/aws-cdk-lib/2.258.0/

For reference, a related pattern apigw-python-cdk-lambda-snapstart documents Python 3.12+ in the same style.
https://github.com/aws-samples/serverless-patterns/tree/main/apigw-python-cdk-lambda-snapstart

Thanks again!