Creating feature cleanup issues shouldn't require a GitHub credential

[emyller]

Is your feature request related to a problem?

Cleanup issues are created via the GitHub Issues API, so Flagsmith must
hold an org-wide write-access PAT — a heavy secret for a one-click outcome.
The issue is also authored by the integration, not the user.

Describe the solution you'd like.

Redirect to GitHub's New Issue page with pre-filled query params, e.g.
https://github.com/Flagsmith/flagsmith/issues/new?title=Test+title&body=Cleanup+prompt.
Repository is already known from code references. No credential needed;
the user submits as themselves.

Describe alternatives you've considered

Keeping the Issues API flow — requires provisioning and rotating a broad
credential.

Additional context

No response