chore: adopt Apache-2.0 license

[skyoo2003]

Summary

Switch KVS from MIT to Apache License 2.0 and add audited third-party notices.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor
• CI/CD

Changes

• Replace the root license with Apache License 2.0 and add NOTICE.
• Inventory runtime Go-module notices and validate the inventory in CI.
• Ship NOTICE in release archives and update release metadata and documentation.

Changelog

• changie new fragment added — one sentence, two at most, 280 characters max
• Not user-visible, no fragment needed

Testing

• make all passes
• New tests added (if applicable)

Related Issues

None.