15.x relnotes: note stable MAC address changes

src 604d34c23f77 was tagged for relnotes, but we indavertently missed
it. Rectify the mistake now and note it in both 15.0/15.1 relnotes,
because 15.1 is a very attractive upgrade target right now and it'd be
nice if we didn't bury it just back in the 15.0 release notes.

PR: 296129
Reviewed by: ziaee
Differential Revision: https://reviews.freebsd.org/D57647