Commit message:
net/http: consolidate multipart form temp file cleanup for all HTTP versions
Historically, we have let each HTTP implementation handle its own
cleanup for any temporary files that might have been created when
handling multipart forms. Now that HTTP/2 has been moved to std, we
might as well consolidate these, and let net/http handle the cleanup
for HTTP/1, HTTP/2, and HTTP/3.
Also re-enable TestParseFormCleanup that was skipped for HTTP/2, and was
silently broken again due to the HTTP/2 bridging creating a new request,
causing the original HTTP/2 cleanup to not work.
For #70914