doc/go1.17: further revise OpenBSD release notes

Simplify and remove forward-compatibility reference, as OpenBSD 6.9 has
already been released (1st of May 2021).

Updates #44513

Change-Id: I0a1abbb397f31d15c80a970edaa9723f894cafa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/327652


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>