website: Restore FreeBSD Project copyright

From 1995-2021, the project website said "1995-???? The FreeBSD Project"
until the rewrite to hugo/asciidoctor, when it became erroneously listed
as 1995-2020 The FreeBSD Foundation. The Foundation owns the trademarks,
not the copyright, and they can not own a copyright before they existed.

Reported by: wosch
Reviewed by: wosch
Event: Berlin Hackathon 202609
Fixes: 989d921f5d4a ("Migrate doc to Hugo/AsciiDoctor")