Pass siteMvn as a String so the site stage runs on Maven 4

[slachiewicz]

siteMvn is passed to doCreateTask whole, while maven is iterated:

for (def mvn in mavens) { ... doCreateTask(os, jdk, mvn, ...) }      // maven: a List is correct
...
doCreateTask( os, jdk, siteMvn, tasks, first, 'site', taskContext )   // siteMvn passed whole

and doCreateTask does jenkinsEnv.mvnFromVersion(os, "${maven}"). A Groovy List stringifies to the literal "[4.0.x]", which matches no case in mvnFromVersion and falls through to default: return 'maven_3_latest'. It is not null either, so the mvnName == null skip never fires — the site stage just quietly runs Maven 3.

maven: [ "4.0.x" ] stays a List; only siteMvn needs to be a String. apache/maven-deploy-plugin already does it this way.

Verified by reading the shared library, not by running a build — the pipeline cannot be executed locally. It also does not make Jenkins green on its own: maven_4_latest still resolves to 4.0.0-rc-5 on the build nodes. This makes the configuration do what it already says.

Same fix as apache/maven-clean-plugin#331.

[slachiewicz]

Confirmed in production — this repo's own master build #171:

OS: linux JDK: 17 Maven: 4.0.x   => Label: ubuntu JDK: jdk_17_latest Maven: maven_4_latest
OS: linux JDK: 21 Maven: 4.0.x   => Label: ubuntu JDK: jdk_21_latest Maven: maven_4_latest
OS: linux JDK: 17 Maven: [4.0.x] => Label: ubuntu JDK: jdk_17_latest Maven: maven_3_latest

The first two lines are the maven: list, correctly iterated to a String. The third is siteMvn passed whole, stringified to [4.0.x], resolved to maven_3_latest — the site stage running on Maven 3, exactly as this PR describes. The workspace directory is even named linux-jdk17-m[4.0.x]_site.

Since the shared library started provisioning Maven through the wrapper, the same value now also reaches -Dmaven=, so it fails outright rather than silently downgrading:

+ mvn ... wrapper -Dmaven=[4.0.x]
wget: Failed to fetch .../apache-maven-[4.0.x]-bin.zip
Failed in branch linux-jdk17-m[4.0.x]_site

The library-side fixes are apache/maven-jenkins-env#8 and apache/maven-jenkins-lib#23.