doc: Remove mention of Git protocol support

doc: Remove mention of Git protocol support

The project Git server hasn't supported cloning with the Git protocol
in a very long time, but the documentation never got the memo. Remove
the mention of using the Git protocol, and while there wrap a mention
of Git in <productname> tags.

Backpatch down to all supported versions.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CABwTF4WMiMb-KT2NRcib5W0C8TQF6URMb+HK9a_=rnZnY8Q42w@mail.gmail.com
Backpatch-through: 13