website: Update supported compilers and Ubuntu versions

For building we now support clang-14 and gcc-12. We also support Ubuntu
22.04 (alongside 20.04 and 18.04).

Change-Id: Id59f56210294af6691a6fcfd5c39b61e86617f37
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/64271
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>