Don't mind pending cluster tasks in docs build
Don't mind pending cluster tasks in docs build

This removes an assertion that the cluster doesn't have any
pending cluster state tasks from the `_cat/health` docs.

Relates to #18160

strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc