Version bump 6.2.1

[jrothrock]

Summary

• Bumps VERSION to 6.2.1
• Moves the pending changelog entries under a 6.2.1 heading and adds brief entries for the other three merged PRs that lacked changelog entries (Fix GraphQL layer leak when unhandled exceptions skip end_execute_field #626 GraphQL layer leak, Honor log level during bootstrap #628 log level bootstrap, Use HEROKU_BUILD_COMMIT for Heroku git revision detection #625 Heroku build commit)

Test plan

• Changelog and version file reviewed for accuracy against the underlying merged PRs

[claude]

Reviewed. Verified all four merged PRs referenced (#624, #625, #626, #628) and confirmed the changelog summaries accurately describe each commit's change. Version bump 6.2.0 → 6.2.1 is a correct patch-level bump for a set of bug fixes only. Formatting matches the existing CHANGELOG conventions (PR references in parentheses, consistent with the #613/#611 entries above). CI is green across the full matrix; no code/tests are needed for a pure version+changelog bump.

No issues found — looks good to merge (note: bot sessions can't submit an "Approve" review, so leaving this as a comment instead).

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