feat: remove the unimplemented flag from UpdateStore request

[rhamzeh]

Description

This removes the unimplemented flag from the UpdateStore method.

Note: I would prefer that this becomes a PUT instead of a PATCH as it currently is, however will leave that up for discussion before contributing that change

References

Goes in tandem with:

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[curfew-marathon]

This PR has had no human activity for 90 days, so it has been marked lifecycle/stale.

This is automated backlog grooming, not a judgment on the work.

What happens next, unless there is activity:

• In 30 days, it will move to lifecycle/closing-soon
• 30 days after that, it will be closed

To keep it open, push a commit or leave a comment, and the clock will reset. For work that should not auto-close, such as an RFC or long-running experiment, ask a maintainer to add lifecycle/frozen.

[curfew-marathon]

This PR has had no human activity for 30 days since being marked lifecycle/stale, so it has now been marked lifecycle/closing-soon.

It will be closed in 30 days unless there is new activity.

If the work still matters, this is the moment to say so: leave a comment, push a commit, or ask a maintainer to add lifecycle/frozen if it should be kept open long-term.

If this work is no longer being pursued, no action is needed.