Get rid of CentOS 7 in doc

Both:

directly including in hammer.py and indirectly with g++ 4.8.5 (doc/sphinx/arm/install.rst, doc/sphinx/arm/hooks-radius.rst and platforms.rst) with perhaps old MySQL too (MySQL has no min version requirement, should we add one),
using CenOS 7 as the base for examples (see above). The new "reference system" should be the last LTS Ubuntu so the 22.04 "Jammy Jellyfish" version (released 2022-04-21, standard support until 2027-04-21, security patches until 2032-04-21).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information