Update to Java 25 (LTS)

[GuusLieben]

Type of Change

• Chore (changes to the build process or auxiliary tools)

Description

With the release of Java 25, which is the latest LTS release at the time of writing, Hartshorn will drop Java 21 as its main JDK target. As such, this PR migrates our tooling to Java 25, and fixes minor issues with compact source files (related to constructorless classes in unnamed packages).

A known limitation of compact source files is the fact that they do not reside within a named package. As such, we cannot perform automatic package scanning unless explicitly declared. The main class itself will always be included, including any bindings it declares.

A valid compact source file may look like the sample below, which is adapted from the Java Launchpad sample starter:

import ...;

void main(String[] args) {
    HartshornApplication.createApplication(args).initialize(configuration -> {
        configuration.scanPackages(packages -> {
            packages.add("org.dockbox.sample.java");
        });
    });
}

@Singleton
public ApplicationStarter applicationStarter() {
    return applicationContext -> {
        GreetingAction greetingAction = applicationContext.get(GreetingAction.class);
        greetingAction.greet();
    };
}

Checklist

• I have performed a self-review of my own code
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have rebased my branch on top of the latest develop branch

[GuusLieben]

See #1153