Let Renovate update the 4.x maintenance branch [skip ci]

[vpaturet]

Summary

Add 4.x to Renovate's baseBranchPatterns so dependency updates are proposed on the new 4.x maintenance branch, as they are on 3.x.

Renovate reads its configuration from the default branch only, so this has to be on master rather than on 4.x.

Context

• 4.x was created from master (4.0.19-SNAPSHOT) ahead of Upgrade to netex2-java-model 3.0.0 (NeTEx 2.0) #328, which moves master to 5.0.0 and netex2-java-model.
• No package rule is needed for 4.x: the old org.entur:netex-java-model artifact stays on 2.0.16.x, so Renovate will only offer patch updates of it there.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.38%. Comparing base (e56f382) to head (dd9185d).
⚠️ Report is 71 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #329      +/-   ##
============================================
- Coverage     89.28%   88.38%   -0.90%     
- Complexity      241      243       +2     
============================================
  Files            16       17       +1     
  Lines           821      835      +14     
  Branches         63       70       +7     
============================================
+ Hits            733      738       +5     
- Misses           63       65       +2     
- Partials         25       32       +7     

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