Adjust `IssueablePolicy` to disallow guests from updating their incidents

What does this MR do and why?

Describe in detail what your merge request does and why.

This merge request negates guest authors' ability to update or reopen their issue if it were promoted to an incident.

This MR is related to #345394 (closed).

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a project as Owner (or Admin/Maintainer)
2. Invite user as a Guest
3. Guest creates an Issue
4. Owner changes Issue Type from Issue to Incident
5. Guest visits the (now) Incident and can no longer close or update their Incident

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.