website: Remove EOLed 12.3-RELEASE
- content/en/releases/_index.adoc Move 12.3-RELEASE from production to unsupported release
- Update avisory and errata template for FreeBSD 12.3 EOL
- Remove unsupported 12.3 from download page
- Sidenav: Remove 12.3 from supported releases
- Remove 12.3 from supported branches/releases
- index: Remove 12.3 from supported releases
Approved by: carlavilla (mentor) gjb (re@)
Differential Revision: https://reviews.freebsd.org/D39417