Upgrade explicit Java versions in pom.xml files that have no Maven model

[jkschneider]

A repository built with Gradle can also carry a pom.xml (spring-petclinic does). The LST parses it as plain XML with no MavenResolutionResult, so UpdateMavenProjectPropertyJavaVersion never sees it: after the migration the Gradle build targets 25 while the Maven build still says 17 and cannot compile sources that use Java 25 APIs. New UpgradeUnmodeledMavenJavaVersion, added to UpgradeJavaVersion, raises explicit numeric java.version and maven.compiler.{release,source,target} properties and maven-compiler-plugin release/source/target settings below the target in a pom.xml that has no Maven marker. Property references and compiler plugins with a custom groupId are left alone (second commit).

Found by a team of coding agents reviewing a Moderne run of org.openrewrite.java.migrate.UpgradeToJava25 over eight open-source repositories (run 20261003103223-WtyA8). The issue is alternate-maven-java-version in the run's io.moderne.RecipeIssues data table, and the fix was discussed and reviewed on the run's message board. A row from the table, as a generalized example:

The Gradle migration leaves the alternate Maven build targeting Java 17 although migrated sources require Java 25. (spring-projects/spring-petclinic, pom.xml)

Before:

<project><properties><java.version>17</java.version></properties></project>

What the recipe produced:

<project><properties><java.version>17</java.version></properties></project>

Expected:

<project><properties><java.version>25</java.version></properties></project>

Stacked on #1251 (3 of 9); merge that one first.