v2.1.0 - v2 Stable Graduation

Stable graduation release. Promotes the v2 line out of alpha and
codifies the criteria future stable releases must meet.

Highlights

• v2 stable graduation
• Documented stability promotion criteria

Added

• docs/STABILITY.md documenting alpha exit policy and stable promotion checks

Changed

• Version string moved from 2.0.4-alpha to 2.1.0

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Full Changelog: v2.0.0-alpha...v2.1.0