chore: Bump version to 1.21.0

Workflow run: https://github.com/HumanSignal/label-studio/actions/runs/17296353917

Author: vladimirheartex

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "label-studio"
-version = "1.22.0.dev0"
+version = "1.21.0"
 description = "Label Studio annotation tool"
 authors = [
     { name = "HumanSignal", email = "[email protected]" },