chore: reconcile lambda runtime support

[ava-silver]

What does this PR do?

• Removes Node.js 12, 14, and 16, Python 3.6 and 3.7, and provided runtime support.
• Updates layer and integration tests to use Python 3.8.
• Verifies that Node.js 16 follows the existing unsupported-runtime path.

Motivation

Align customer-visible runtime acceptance with the approved Lambda runtime catalog.

Testing Guidelines

Not applicable.

Additional Notes

This PR is stacked on the Lambda layer catalog format-preparation PR (#685).

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[ava-silver]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [FEAT] add runtime testing escape hatch #687
• chore: reconcile lambda runtime support #686 👈 (View in Graphite)
• chore: prepare lambda layer catalog #685
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.