chore: release 1.21.0

[vgvbot]

🚨 There are changes ready for release 🚀

ℹ Merge this PR once the team confirms the release is ready.

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1.21.0 (2026-08-19)

Features

• add spell checking on PR titles (#468) (f69487f)
• deps: upgrade to Flutter 3.47 and Dart 3.13 (#486) (bab4f13)

Bug Fixes

• correct typos in issue templates (#484) (a4ab3e1)

Miscellaneous Chores

• bump eslint from 10.6.0 to 10.8.1 in /site (#477) (b3c2ae1)
• bump eslint-plugin-jest from 29.16.0 to 29.16.1 in /site (#488) (c65e189)
• bump globals from 17.7.0 to 17.9.0 in /site (#478) (ee403af)
• bump globals from 17.9.0 to 17.11.0 in /site (#487) (3a295b5)
• bump prettier from 3.9.5 to 3.9.6 in /site (#479) (5aed557)
• bump react-dom from 19.2.7 to 19.2.8 in /site (#476) (58d92ba)
• bump streetsidesoftware/cspell-action from 8 to 9 (#489) (f053008)

Docs

• add Platform.environment guide for dart_package workflow (#473) (61058dd)

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