bump cylc-flow dependency (#771)

* Ensures that new data elements added in cylc/cylc-flow#7138
  are available for the bundled UI version which will include cylc/cylc-ui#2378

Author: oliver-sanders

setup.cfg

@@ -45,7 +45,7 @@ install_requires =
     # dependencies first. This way, if other dependencies (e.g. jupyterhub)
     # don't pin versions, we will get whatever cylc-flow needs, and not the
     # bleeding-edge version.
-    cylc-flow==8.6.*
+    cylc-flow>=8.6.2.dev,<8.7
     ansimarkup>=1.0.0
     jupyter_server>=2.13.0
     requests