doc: document that FreeBSD 9 support will be removed in Go 1.10

[rsc]

Maybe I missed a decision, but I thought we needed to document in Go 1.9 release notes that Go 1.10 will remove support for FreeBSD 9. I don't see that there.

[bradfitz]

Go 1.8 works on FreeBSD 9.3.
Go 1.8 does NOT work on FreeBSD 9.1 (unless CGO_ENABLED=0).

We didn't discover this until recently (#20850 and 25de52e), because we ran FreeBSD 9.3 builders, not FreeBSD 9.1.

But sure, we can say that Go 1.10 will remove FreeBSD 9.3 support entirely, but I don't know why we would. I don't recall hearing any benefits of doing so.

[emaste]

For reference, FreeBSD releases now unsupported by the FreeBSD security team can be found at https://www.freebsd.org/security/unsupported.html. All 9.x releases are on the list.

Even so, I am sure there are users still on 9.3 that would appreciate being able to run newer Go still.

[gopherbot]

CL https://golang.org/cl/47731 mentions this issue.