Bump Jenkins version to 2.426.3

[michael-doubez]

Based on version extract, it seems safe to bump Jenkins version to 2.426.3.
At the same time, update JCasc as indictated in #327

Also prepare for jetty 12 as requested in JENKINS-73141

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.59%. Comparing base (bca3705) to head (31c64a9).
Report is 10 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #346      +/-   ##
============================================
+ Coverage     72.46%   73.59%   +1.12%     
- Complexity      201      209       +8     
============================================
  Files             9       10       +1     
  Lines           839      852      +13     
  Branches        119      120       +1     
============================================
+ Hits            608      627      +19     
+ Misses          170      163       -7     
- Partials         61       62       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.