Transport.DialerAndTLSConfigSupportsHTTP2 was added just earlier
in CL 130256 but we thought of a better name moments after submitting.
ForceAttemptHTTP2 is shorter, more direct, and doesn't constrain what
we can use it with in the future.

Updates #14391
Updates #27011

Change-Id: Ie5fc71bafcbcaa1941b5d49f748b6d710503d477
Reviewed-on: https://go-review.googlesource.com/c/go/+/172299Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>