[skip ci] Update changelogs for v0.21.0-rc.1

GitOrigin-RevId: 1fef910774445c2da65c34aa8682b03b636c7bd7

Author: snpefk

CHANGELOG.md

@@ -4,6 +4,8 @@ Mapbox welcomes participation and contributions from everyone.
 
 ## main
 
+## 11.21.0-rc.1 - 23 March, 2026
+
 ## 11.20.1 - 17 March, 2026
 
 ## 11.20.0 - 13 March, 2026