cmd/compile,cmd/go: fix long test builders

CL 361411 improved error message for go version requirement, but forgot
to update the test in cmd/go to match new error message. That causes
longtest builders failed.

This CL changes mod_vendor_goversion.txt to match compiler error, and
limit fixedbugs/issue49368.go to run with -G=3 only.

Updates #49368

Change-Id: I125fe0a8c2a1595066d39c03e97819e7a1274e0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/361963


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>