Flagsmith dashboard should use the new consolidated endpoint for updating a flag

As part of the versioning migration work, we have created 2 new endpoints for updating flags as per the documentation [here](https://docs.flagsmith.com/integrating-with-flagsmith/flagsmith-api-overview/admin-api/updating-flags).

We would like to update the Flagsmith dashboard to use these endpoints instead of the current approach which uses different endpoints depending on whether the environment uses v2 versioning or not.

The goal of this issue is:

1. Simplify the FE code
2. Identify any limitations in the new endpoints
3. Identify if we need both endpoints, or if one structure is favourable over the other

Once this issue is complete, we can share these endpoints with the public as part of the versioning migration process.