Deprecate keep_warm in favor of AWS-native cold start solutions

[monkut]

Summary

• Change keep_warm default from true to false
• Add deprecation warning during deploy/update/schedule when keep_warm is enabled
• Add DeprecationWarning in keep_warm_callback handler
• Update README documentation to mark keep_warm as deprecated

Context

Investigation in #1445 confirmed that keep_warm:

• Does not reduce cold start duration — init times are identical (~1,050ms) with or without it
• Only reduces cold start frequency by keeping one execution environment alive
• Only keeps 1 environment warm — concurrent requests still hit full cold starts

AWS-native alternatives are superior:

• SnapStart: 45-72% faster cold starts
• Provisioned Concurrency: eliminates cold starts entirely

The feature remains functional for users who explicitly opt in. Full removal planned for a future major version.

Closes #1451

Test plan

• New tests verify deprecation warning appears when keep_warm: true
• New tests verify no warning when keep_warm: false
• New tests verify keep_warm_callback emits DeprecationWarning
• New tests verify default is now false
• Pre-commit hooks pass (isort, black, flake8, mypy)
• Existing test suite unaffected

[coveralls]

coverage: 74.692% (+0.04%) from 74.651% — deprecate-keep-warm into master