Prepare release 1.6.0

Workflow: Release stage 1 - create release PR, run: 22

Author: github-actions[bot]

CHANGES.md

@@ -11,7 +11,7 @@ creating a new release entry be sure to copy & paste the span tag with the
 updated. Only the first match gets replaced, so it's fine to leave the old
 ones in. -->
 -------------------------------------------------------------------------------
-## __cylc-ui-1.6.0 (<span actions:bind='release-date'>Upcoming</span>)__
+## __cylc-ui-1.6.0 (<span actions:bind='release-date'>Released 2023-04-27</span>)__
 
 ### Enhancements