docs: document and finalize Gin v1.12.0 release (#4551)

* docs: document and finalize Gin v1.12.0 release

- Add changelog entries for the Gin v1.12.0 release, covering new features, enhancements, bug fixes, build updates, and dependency upgrades
- Update all historical changelog entries to consistently use the same bullet style
- Bump Gin version constant to v1.12.0
- Update README to announce Gin 1.12.0 instead of 1.11.0

Signed-off-by: appleboy <appleboy.tw@gmail.com>

* Update CHANGELOG.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Signed-off-by: appleboy <appleboy.tw@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: appleboy