lang/go123: Update to 1.23.12

While here: golang.org now points to go.dev (thanks to Meng Zhuo in the
Review below).

Changes:
  go1.23.12 (released 2025-08-06) includes security fixes to the
  database/sql and os/exec packages, as well as bug fixes to the
  runtime. See the Go 1.23.12 milestone (follows) on our issue tracker
  for details.

  https://github.com/golang/go/issues?q=milestone%3AGo1.23.12+label%3ACherryPickApproved

Differential Revision:	https://reviews.freebsd.org/D50595