…shut down in their API (#13271)

## Background

xAI has shut down the Grok 2 models (`grok-2-vision-1212`,
`grok-2-vision`, `grok-2-vision-latest`, `grok-2-image`,
`grok-2-image-1212`, `grok-2-image-latest`) in their API. Keeping these
model IDs in the SDK causes confusion and sets users up for API errors.

While #13016 only mentions a few of them, they are in fact all shut
down, which I verified - some of them were only aliases in our codebase
anyway. Also see
https://github.com/vercel-labs/provider-models-monitor/blob/main/provider-models/xai.json
for reference - no more Grok 2 entirely.

## Summary

- Removes all Grok 2 model IDs from `XaiChatModelId` and
`XaiImageModelId`, and updates all examples, docs, and tests that
referenced them to use their Grok 3 / `grok-imagine-image` successors
instead.
- Removes `xai/grok-2-vision` from the gateway model ID list. cc @shaper

## Manual Verification

N/A

## Checklist

- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Future Work

N/A

## Related Issues

Fixes #13016