Update error message for SSH transport plugin

[khsrali]

Fix #7109

Update the error message of core.ssh when sftp is not allowed. In this case, we recommend user to setup a computer using core.ssh_async plugin with openssh backend.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.58%. Comparing base (cd11f08) to head (b8bb903).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7110   +/-   ##
=======================================
  Coverage   79.58%   79.58%           
=======================================
  Files         566      566           
  Lines       43517    43517           
=======================================
  Hits        34629    34629           
  Misses       8888     8888           

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