Fixed confluence add_user_group to use right URL

[Mauwork]

Rel: #1422
The URL of the confluence REST API was wrong in add_user_group and it has to be a PUT request according to:

https://docs.atlassian.com/ConfluenceServer/rest/8.2.0/#api/user/{username}/group/{groupName}-update

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 34.38%. Comparing base (42ed0b0) to head (710fe28).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
atlassian/confluence.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1425   +/-   ##
=======================================
  Coverage   34.38%   34.38%           
=======================================
  Files          46       46           
  Lines        8586     8584    -2     
  Branches     1601     1601           
=======================================
  Hits         2952     2952           
+ Misses       5511     5509    -2     
  Partials      123      123           

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