Reword preview warning for `target-version` Python 3.14 (#19563)
Small rewording to indicate that core development is done but that we
may add breaking changes.
Feel free to bikeshed!
Test:
```console
❯ echo "t''" | cargo run -p ruff -- check --no-cache --isolated --target-version py314 -
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.13s
Running `target/debug/ruff check --no-cache --isolated --target-version py314 -`
warning: Support for Python 3.14 is in preview and may undergo breaking changes. Enable `preview` to remove this warning.
All checks passed!
```
