Replace CentOS 7 references with EL8 (#441)

* Update QIG with EL8 and remove centos 7

* remove remaning mentions of centos 7

* Update _requirements.rst

* Update qig.rst

* Update qig.rst

* Update qig.rst

* Update qig.rst

* nmcli commands and corrections

* Update qig.rst

* Update qig.rst

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Co-authored-by: Andrija Panic <45762285+andrijapanicsb@users.noreply.github.com>