Creating feature cleanup issues shouldn't require a GitHub credential

### 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.
.
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_

Start at the existing cleanup-issue creation flow and its GitHub Issues API call, then trace where the repository, title, and cleanup prompt are assembled. Replace that submission path with GitHub's pre-filled New Issue URL, preserving the displayed title and body. Done means users can open and submit the cleanup issue as themselves without a stored GitHub credential.

Written by the indexing model from the issue text.