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>

doc/go1.17.html

@@ -106,8 +106,7 @@ <h3 id="openbsd">OpenBSD</h3>
  of directly using machine instructions. In Go 1.17, this is also
  done on the 32-bit x86 and 32-bit ARM architectures on OpenBSD
  (the <code>openbsd/386</code> and <code>openbsd/arm</code> ports).
- This ensures forward-compatibility with future versions of
- OpenBSD, in particular, with OpenBSD 6.9 onwards, which requires
+ This ensures compatibility with OpenBSD 6.9 onwards, which require
  system calls to be made through <code>libc</code> for non-static
  Go binaries.
 </p>