website/i18n: make 'Legacy Release' an i18n keyword
This text appears in the 'sidenav' component on the left side at:
https://www.freebsd.org/releases/
Currently it is hard-coded. Add the keyword to the translation
templates.
Reviewed by: vladlen, ziaee
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56182