update changelog for version 2.21

Changelog.md

 # Changelog
 
+## [2.21] - 2024-10-22
+- Fix inventory structure
+- Fix the `pe_router_list` in update_sdp_single_pe
+- Remove --concurrency=1 --pool=solo
+- Don't fail router termination if the device doesn't exist in LibreNMS or Kentik
+- Improve error handling for missing LibreNMS device in router termination
+- Add moodi start and stop steps
+- Add pytest-xdist for parallel test execution
+- Update validate_router WF_USABLE_MAP to run validations only on "ACTIVE" routers, allowing workflows for provisioning routers.
+
 ## [2.20] - 2024-10-10
 -  Fix callback step live update by bumping to orchestrator-core==2.7.6