[release] Bumped to release 9.7.4

[Aryamanz29]

Summary

• Bumps pyatlan from 9.7.3 → 9.7.4 (patch).
• Adds a 9.7.4 entry to HISTORY.md.

Release notes (pre-pended to HISTORY.md)

## 9.7.4 (June 2, 2026)

### Experimental: `pyatlan_v9`

- Generated latest typedef models. (#947)

Test plan

• pyatlan/version.txt reads 9.7.4
• HISTORY.md prepended with the 9.7.4 entry above (older releases untouched)
• Single [release] commit on a dedicated bump-to-release-9.7.4 branch
• CI green
• After merge: tag 9.7.4 will be created via the release flow (no manual tag in this PR)

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