chore: remove obsolete GPT-4 PR guidance

[extrasmall0]

Summary

• remove obsolete GPT-4 access language from the PR template
• replace it with generic guidance about documenting results on currently available baseline models
• keep the rest of the contribution checklist intact

Testing

• docs/template only

Closes #1608

[extrasmall0]

Quick status refresh after rechecking this against the current docs flow: this is still a docs-only cleanup removing obsolete GPT-4 PR guidance that no longer matches how contributions are handled today. There is no evaluator logic, benchmark behavior, or runtime change in this PR. If you would rather fold the wording into a broader docs sweep or trim it further, I am happy to update it.