examples: remove `next-offline` example (#73675)
## What?
Remove [`next-offline`
example](https://github.com/vercel/next.js/tree/canary/examples/next-offline).
## Why?
[`next-offline`](https://github.com/hanford/next-offline) package hasn't
been maintained in 3 years.
### Adding or Updating Examples
- [x] The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
- [x] Make sure the linting passes by running `pnpm build && pnpm lint`.
See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md
CC: @samcx
Co-authored-by: Sam Ko <sam@vercel.com>
