Declare that Python 3.14 is not supported

[zsol]

Which issue(s) does this change fix?

Until aws/serverless-application-model#3831 gets resolved, this PR declares that aws-sam-cli doesn't support Python 3.14.

Why is this change necessary?

This change helps tools avoid trying to run aws-sam-cli using Python 3.14.

How does it address the issue?

For example when running uvx --from aws-sam-cli sam local --help, uv will read this metadata to determine the best version of Python to use. This PR will help uv understand to not try 3.14.

What side effects does this change have?

N/A

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.