cmd/releasebot: check for Release History entry only for Go 1.12
cmd/releasebot: check for Release History entry only for Go 1.12

The Release History pages have moved to x/website in CL 210797.
Checking that the current release has already been documented should
no longer happen as part of -mode=prepare for Go 1.13.x and newer.

We can consider moving this type of check into a new mode that is to be
run after a release is finished. That is discussed in golang/go#36086.

Fixes golang/go#36075
Updates golang/go#36086

Change-Id: I7547ddd4032e970ac7af1572a98698868d936a48
Reviewed-on: https://go-review.googlesource.com/c/build/+/210957
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

golang/build,golang/build,golang/build,golang/build