[4.0.x] [GH-11772] Fail-fast consumer POM validation for non-4.0.0 model versions

[gnodet]

Cherry-pick of #11780

Adaptation for 4.0.x:

• Removed deployBuildPom() method (constant not yet on 4.0.x)
• Added missing MavenException import
• Added super("[4.0.0-rc-1,)") constructor to IT test

[github-actions]

@gnodet Please assign appropriate label to PR according to the type of change.

[ascheman]

Real-world adopter feedback — tested apache/plc4x against a current 4.0.x SNAPSHOT (pre-RC-6). Posting here since this backport first ships with RC-6.

The fail-fast works as designed: PLC4X's parent POM (model 4.1.0, using <subprojects> and build <sources> — the latter on this PR's non-strippable list) now stops the very first reactor module on install, where rc-5 "succeeded" but silently installed a model-4.1.0 consumer POM — exactly the #11772 poison. Root-module matrix:

Maven	mvn -N install
4.0.0-rc-5	SUCCESS (but consumer POM stays 4.1.0)
4.0.x-SNAPSHOT	fail-fast ✔
4.0.x-SNAPSHOT + maven.consumer.pom.flatten=true	SUCCESS

With the flatten property in .mvn/maven.config we then ran a full A/B (identical sources, separate local repos, tests on): rc-5 and the SNAPSHOT are behaviorally identical — 113 modules, 4058/4058 tests green on both, identical installed file lists (156), identical consumer-POM model distribution per artifact (28× self-contained 4.0.0, 1× 4.0.0 with parent, 2× 4.1.0 with parent for the pom-packaged aggregators). So the fail-fast is the only rc-5 → RC-6 behavior change here, and the suggested remedy is safe on both versions.

Three observations that may be worth acting on before RC-6:

1. Release notes: our Maven 4 adoption registry currently tracks ~70–85 GitHub projects declaring the 4.1.0 model (report). Every one of them that installs/deploys a parent POM with non-strippable 4.1.0 features will hard-fail on RC-6 upgrade. A release-notes callout with the three remedies from the error message would soften the landing considerably.

2. The error text slightly over-promises for pom packaging: option 1 says enabling flattening will "produce a self-contained 4.0.0 consumer POM", but with flattening enabled the pom-packaged project's own consumer POM is still published as model 4.1.0 with <parent> and <subprojects> (identically on rc-5, so not a regression). Children are correctly flattened and no longer reference it — but direct consumers inheriting from such a parent would still hit Consumer POM of parent POMs currently remain at modelVersion 4.1.0, breaking Maven 3 and Gradle consumers #11772-style breakage. If "handles what it can" is the intended semantics, maybe adjust the error text and document the residual limitation.

3. As a data point for how adopters need to react: PLC4X is now prepared via Enable consumer POM flattening for Maven 4 RC-6 readiness plc4x#2647 (one line in .mvn/maven.config, verified against rc-5 and the SNAPSHOT).

Happy to re-run this against RC-6 staging when it's cut.