Updates cms-sanity example to use Sanity v3 (#45662
## Bug
- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
Fixes issues reported, for instance here:
https://github.com/vercel/community/discussions/1449
(The example does not deploy atm and uses an old version of Sanity).
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [x] Related issues linked using `fixes #number`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`]
* Updates the Sanity studio to use the latest v3 version. This required
changing some files, as per migrating to Studio v3 docs.
* Updated the README to reflect the changed procedure.
* Update the next app to use the latest version of next-sanity. Changed
related code as a result.
### Testing
Tested using this deploy url (uses the branch from the forke repo):
https://vercel.com/new/clone?repository-url=https%3A%2F%2Fgithub.com%2Fsanity-io%2Fnext.js%2Ftree%2Fsanity-v3%2Fexamples%2Fcms-sanity&repository-name=cms-sanity&project-name=cms-sanity&demo-title=Blog+using+Next.js+%26+Sanity&demo-description=On-demand+ISR%2C+sub-second+as-you-type+previews&demo-url=https%3A%2F%2Fnext-blog-sanity.vercel.app%2F&demo-image=https%3A%2F%2Fuser-images.githubusercontent.com%2F110497645%2F182727236-75c02b1b-faed-4ae2-99ce-baa089f7f363.png&integration-ids=oac_hb2LITYajhRQ0i4QznmKH7gx
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
fixes #46450
