[WolfTasks]

Summary

• Compile the backend for Java 25 (sourceCompatibility, Kotlin jvmTarget).
• Use JDK 25 in the ci, codeql and docker-publish workflows.
• Update the stale comment in backend/Dockerfile and the JDK requirement in mkdocs/development.md.

The runtime image already ran on Temurin 25 while builds and CI used 21, so the shipped JVM was not the one under test.

Verification

Run locally:

• Backend tests on Temurin 25.0.5: 414 of 414 passed (same count on 21.0.13 before the change).
• Compiled classes have major version 69 (Java 25).
• bootJar and the Docker image build.
• The image starts on Java 25 and stops at the database connection, because none was provided. A start against a real database was not run.

To check on this PR

• CodeQL java-kotlin is blocking and could not be run locally. If it reports Java 25 as too recent, bump codeql-action.

Known, not changed here

• The base image digest is pinned to JRE 25.0.4.1, older than the current 25.0.5.
• ByteBuddy calls a sun.misc.Unsafe method that is due for removal, which logs a warning during tests.

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