[DOCS] gitfs walkthrough references out of date dependencies

[mpounsett]

Description
The Git Fileserver documentation is out of date, still referring to dependency requirements on long unsupported operating systems (e.g. Ubuntu 14 and Debian Wheezy).

Suggested Fix
Instructions and requirements should be updated to match current requirements and supported software.

Type of documentation
Salt documentation

Location or format of documentation
https://docs.saltproject.io/en/latest/topics/tutorials/gitfs.html

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