• Update the 12.1-RELEASE schedule to reflect releng/12.1 had branched, and the BETA1 builds are in flight.
• Document __FreeBSD_version numbers 1201000 and 1201500.
• Add releng/12.1 to the list of branches in the releng page.

Approved by: re (implicit)
Sponsored by: Rubicon Communications, LLC (Netgate)