MinimumRequirements: drop requirements for unsupported Go versions

The full historical requirements are documented on the individual GOOS
pages, except for Darwin, which was out of date.

The current MinimumRequirements and Darwin pages disagree whether 10.14+
requires Go 1.11 or 1.12. I've gone with the more conservative option,
1.12.

Change-Id: I6a6a636cd965f6b3ea35b6e4b6d0d932a1c6f2b8
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/746603
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>