crypto/rsa: allow keys larger than 16384 bits in FIPS mode

crypto/rsa: allow keys larger than 16384 bits in FIPS mode

Nothing in the standard enforces an upper limit, and we can try
documenting an open range in the Security Policy. Worst case, this is
easy to revert.

For #69536

Change-Id: Id3082e73556fdcd6d2e6c2054c512516e9156c5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/632536
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>