Fix 2.6 regression on group field and deprecate Jackson parser (fix #236)

[michael-doubez]

In 2.6, introduction of group field splitting caused a regression when loading config from XML (see #236).

At the same time, deprecating usage of Jackson2 for using GSon everywhere since it is used by google api.

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[codecov]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 71.90%. Comparing base (22331f0) to head (657a4f5).
Report is 3 commits behind head on master.

Files	Patch %	Lines
...va/org/jenkinsci/plugins/oic/OicSecurityRealm.java	52.94%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #280      +/-   ##
============================================
- Coverage     71.90%   71.90%   -0.01%     
- Complexity      194      196       +2     
============================================
  Files             9        9              
  Lines           744      751       +7     
  Branches        124      126       +2     
============================================
+ Hits            535      540       +5     
- Misses          150      151       +1     
- Partials         59       60       +1     

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