Add changelog for 1.21.9 (#29971)

Co-authored-by: Jason Song <i@wolfogre.com>

CHANGELOG.md

@@ -4,6 +4,39 @@ This changelog goes through all the changes that have been made in each release
 without substantial changes to our git log; to see the highlights of what has
 been added to each release, please refer to the [blog](https://blog.gitea.com).
 
+## [1.21.9](https://github.com/go-gitea/gitea/releases/tag/1.21.9) - 2024-03-21
+
+* PERFORMANCE
+  * Only do counting when count_only=true for repo dashboard (#29884) (#29905)
+  * Add cache for dashboard commit status (#29932)
+* ENHANCEMENT
+  * Make runs-on support variable expression (#29468) (#29782)
+  * Show Actions post step when it's running (#29926) (#29928)
+* BUGFIXES
+  * Fix PR creation via API between branches of the same repo with head field namespaced (#26986) (#29857)
+  * Fix and rewrite markup anchor processing (#29931) (#29946)
+  * Notify reviewers added via CODEOWNERS (#29842) (#29902)
+  * Fix template error when comment review doesn't exist (#29888) (#29889)
+  * Fix user id column case (#29863) (#29867)
+  * Make meilisearch do exact search for issues (#29740 & #29671) (#29846)
+  * Fix the `for` attribute not pointing to the ID of the color picker (#29813) (#29815)
+  * Fix codeowner detected diff base branch to mergebase (#29783) (#29807)
+  * Fix Safari spinner rendering (#29801) (#29802)
+  * Fix missing translation on milestones (#29785) (#29789)
+  * Fix user router possible panic (#29751) (#29786)
+  * Fix possible NPE in ToPullReviewList (#29759) (#29775)
+  * Fix the wrong default value of ENABLE_OPENID_SIGNIN on docs (#29925) (#29927)
+  * Solving the issue of UI disruption when the review is deleted without refreshing (#29951) (#29968)
+  * Fix loadOneBranch panic (#29938) (#29939)
+  * Fix invalid link of the commit status when ref is tagged (#29752) (#29908)
+  * Editor error message misleading due to re-used key. (#29859) (#29876)
+  * Fix double border and border-radius on empty action steps (#29845) (#29850)
+  * Use `Temporal.PlainDate` for absolute dates (#29804) (#29808)
+  * Fix incorrect package link method calls in templates (#29580) (#29764)
+  * Fix the bug that the user may log out if GetUserByID returns unknown error (#29962) (#29964)
+  * Performance improvements for pull request list page (#29900) (#29972)
+  * Fix bugs in rerunning jobs (#29983) (#29955)
+
 ## [1.21.8](https://github.com/go-gitea/gitea/releases/tag/1.21.8) - 2024-03-12
 
 * SECURITY