Remove LATEST_SUPPORTED_ADMIN_VERSION and RELEASE_CANDIDATE_ADMIN_VERSION constants

These constants caused unintended breaking changes every quarter when API versions were updated, violating semantic versioning principles.

Developers must now explicitly specify API versions in Context.setup, giving them control over when to upgrade and test new versions. See BREAKING_CHANGES_FOR_V15.md for migration guide.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>