Upgrade to Java 25

[nscuro]

Current Behavior

We build for Java 21, but ship our container images with Java 25 runtime. This means we cannot use Java 25 language features, and cannot strictly rely on Java 25 runtime features being present.

Proposed Behavior

Upgrade the code base to require Java 25, and raise the language level to 25.

Checklist

• I have read and understand the contributing guidelines
• I have checked the existing issues for whether this enhancement was already requested