Opt into Github issue customization

[pajod]

Goal: Gracefully offload some work to reporter for as long as incoming reports exceed triage capacity.

Current: New issue redirects directly to the blank form Suggested Change: Clicking the "New issue" button offers choice

• one of two predefined forms with guidance what to include in such a report
• the (entirely optional) link to the blank form
• and possibly also a 4th option for https://github.com/benoitc/gunicorn/security - pending #3106

Alternative considered: Close more issues #wontfix or #incomplete (often misunderstood as being unfriendly)

Reference: https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests