Geo: update geo_node_name to geo_site_name in gitlab.rb

Problem to solve

!65969 (comment 623962469)

• Update the terminology in gitlab.rb
• Follow this doc https://docs.gitlab.com/omnibus/settings/gitlab.yml.html
• We must support geo_node_name and geo_site_name both for a while and go through the deprecation/removal process.

Also do the same for charts: global.geo.nodeName => global.geo.siteName

How to contribute to the project 💡

• How to contribute to the GitLab project (development)
• Once you set up your GDK, please refer to this document to configure it to the Geo sites.