Reject multi-line f-string elements before Python 3.12 (#24355)
## Summary
Before Python 3.12, a replacement field in an f-string can span multiple
lines only if the outer f-string is triple-quoted. This was relaxed in
Python 3.12, but we weren't rejecting these as syntactically invalid on
earlier versions.
Closes https://github.com/astral-sh/ruff/issues/24348.
