Bump JaCoCo to >= 0.8.13 and add Java 25 to the CI matrix

[ahmed-sekka]

Running the test suite on JDK 25 fails inside the JaCoCo agent:

Caused by: java.lang.IllegalArgumentException: Unsupported class file major version 69

This is fixed in JaCoCo 0.8.13+. The parent POM currently pins 0.8.12.

Acceptance criteria

• Bump jacoco-maven-plugin to >= 0.8.13 in the parent POM
• Add 25 to the CI matrix in .github/workflows/build.yml (next to 17 and 21)
• CI green on all three matrix entries

Why

Java 25 is a current LTS line. Adding it to CI catches forward-compat issues early. Today, contributors on JDK 25 hit the JaCoCo error on the first build.