Add support for python 3.8 deprecation warning and unsupported error

[amazimbe]

What are you trying to accomplish?

Adds changes to set python 3.8 as deprecated and eventually unsupported. The deprecation warning and unsupported error are controlled by the following feature flags:

• python_3_8_deprecation_warning
• python_3_8_unsupported_error

Feature Flags PR: https://github.com/github/dependabot-api/pull/5763

This mirrors the approach used for Bundler v1, composer 1 etc allowing us to guide users towards migrating to python 3.13 while providing a controlled rollout.

How will you know you've accomplished your goal?

We will monitor logs and metrics to ensure:

• Python 3.8 users see the deprecation warning when the feature flag is enabled.
• Actions using python 3.8 are blocked when the unsupported error flag is enabled.
• Tests have been added to validate the behaviour of both feature flags.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[kbukum1]

LGTM

[kbukum1]

@amazimbe
There should be a follow up PR that will integrate the deprecation/unsupported into core that will send or add information for PR or alerts similar to how we do for package manager deprecation and unsupported.