ci: migrate to mavenStage() from jenkins-lib-common

[gjed]

What

Replace hand-rolled Maven stages with mavenStage() from jenkins-lib-common@1.5.0.

Why

Reduce boilerplate. mavenStage handles credentials wrapping, JaCoCo auto-detection, SonarQube gating, and test recording automatically.

Changes

• Replace Build jar, Unit tests, Integration tests, Coverage, SonarQube analysis stages with single mavenStage(splitTests: true, postBuildScript: '...')
• Remove SKIP_TESTS and SKIP_CHECKS pipeline parameters (superseded by mavenStage)
• postBuildScript copies fatjar to package/ for downstream buildPackages/dockerStage

Profile

Auto-detect matches current behavior: -P dev on branch, -P prod on tag.

Behavior notes

• junit recording now active (was missing before)
• recordCoverage same logic (JaCoCo auto-detected from POM)
• SonarQube gating unchanged (devel + PR-*)
• No deploy stage (unchanged)

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

This PR migrates the Jenkins pipeline from custom, hand-rolled Maven build/test/coverage/Sonar stages to the mavenStage() helper from jenkins-lib-common@1.5.0, aiming to reduce boilerplate while preserving existing CI behavior.

Changes:

• Replace multiple Maven-related stages (build, unit/integration tests, coverage, Sonar) with a single stage('Maven') using mavenStage(splitTests: true, postBuildScript: ...).
• Remove SKIP_TESTS and SKIP_CHECKS pipeline parameters (delegating behavior to mavenStage).
• Move fatjar copy logic into mavenStage’s postBuildScript for downstream packaging/docker steps.

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