delete entire flow group with new mutation

[whimsicallyson]

PR Checklist:

• add a short description of what's changed to the top of the CHANGELOG.md
• add/update tests (or don't, for reasons explained below)

Describe this PR

Remove (confusing) ability to delete individual flow versions, replace deleting flow group with new delete_flow_group mutation. Solves #686

[cicdw]

I don't see a benefit at this time to removing the ability to delete a single version -- it's possible some users have relied on this for deleting really old versions. I would prefer if we kept the original logic, but for the "delete all versions" we call this new route instead, at least until we actually remove versions in the backend

[znicholasbrown]

LGTM @whimsicallyson - just 1 comment!

[znicholasbrown]

Thanks for the changes @whimsicallyson