releng: omit branch data for 10.x and earlier
stable/10 was EOL as of October 31, 2018 and stable/9 as of December 31,
- Neither has had a commit since 2021. Trim old entries so that
this table won't grow without bound.
stable/11 is EOL but was last updated on September 7, 2023 so it is
reasonable to keep it in the list for now.
Reviewed by: cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42761