www.freebsd.org/releng: Mention releng/12.2 and releng/11.4

• releng/12.2 branch is created on base r365618
• releng/11.4 is handed over to so@ in base r362501

Approved by: bcr
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26433