gitaly: Deprecate git_data_dirs completely

Context

#7753 deprecated the path field of each entry in git_data_dirs, since the path is now configured under gitaly['configuration']['storage']. The rest of the fields (gitaly_address and gitaly_token) in git_data_dirs can also be configured under gitlab_rails['repositories_storages'].

This means that git_data_dirs serves no purpose and only makes the documentation and configuration more confusing.

Proposal

Remove git_data_dirs completely following the approach in this comment:

1. Deprecate git_data_dirs in 17.6, with a removal date of 18.0. This includes this MR, plus putting it in https://docs.gitlab.com/ee/update/deprecations.html, plus announcing it prominently in the blog post.
2. Change all the documentation to use gitaly['configuration']['storage'] and gitlab_rails['repositories_storages'] instead of git_data_dirs.
   • For single-node instances, users need to use only gitaly['configuration']['storage'].
   • For Gitaly-on-a-separate-node instances, users use gitaly['configuration']['storage'] in the Gitaly nodes and gitlab_rails['repositories_storages'] in the Rails nodes to have proper separation of concerns.
3. Ensure GitLab.com and GET uses the new settings instead of git_data_dirs.
4. Drop usage of git_data_dirs in 18.0.