docs: remove experimental @next/routing note (#94903)
### What?
Remove outdated experimental status notices for `@next/routing` from the
adapters documentation and package README.
### Why?
`@next/routing` is no longer experimental, and the existing notices
incorrectly tie its stabilization to the adapters API.
### How?
Delete the obsolete notices while leaving the routing API documentation
unchanged.
### Verification
- `pnpm prettier --with-node-modules --ignore-path .prettierignore
--write
docs/01-app/03-api-reference/07-adapters/05-routing-with-next-routing.mdx
packages/next-routing/README.md`
- Repository lint-staged checks passed during commit
- Not run: runtime tests (documentation-only change)
<!-- NEXT_JS_LLM_PR -->
