[1pt] Need to remove CentOS 6 from feather packages available list

[avtikhon]

As CentOS 6 was removed from packages builds with commits to Tarantool:
tarantool/tarantool@cb9174e
tarantool/tarantool@8b08a3b

then need to document that CentOS 6 package are not available since 2.8.0-31-g8b08a3ba7 Tarantool version

[NickVolynkin]

Hi @avtikhon! I couldn't find any place in the docs where we have a list of supported OS versions. Do you know of any such list?

UPD: looks like we should make this page, see #1752.

[Totktonada]

https://www.tarantool.io/en/download/os-installation/rhel-centos/

[Onvember]

removed blocks "For RHEL/CentOS 6:" from https://www.tarantool.io/en/download/os-installation/rhel-centos/