Upgrade to pac4j 6.x, require Java 17 or newer, and migrate to EE 9

[basil]

Passes mvn clean verify and ATH's plugins.OicAuthPluginTest, but I am not very familiar with this plugin so I do not know if there is additional work that needs to be done. I am not planning on working on this PR any further, so please treat this PR as a starting point if and when it is desirable to complete this migration.

Testing done

• mvn clean verify
• ATH's plugins.OicAuthpluginTest

[codecov]

Codecov Report

Attention: Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.08%. Comparing base (2a120e0) to head (1b25d1e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/org/jenkinsci/plugins/oic/OicSecurityRealm.java	96.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #455      +/-   ##
============================================
+ Coverage     71.65%   72.08%   +0.43%     
- Complexity      221      223       +2     
============================================
  Files            17       17              
  Lines          1030     1046      +16     
  Branches        146      146              
============================================
+ Hits            738      754      +16     
  Misses          203      203              
  Partials         89       89              

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[fcojfernandez]

I've got a WIP branch with this same effort (except on the jdk-17 stuff), but I didn't have the bandwidth to complete it