[amiliosc]

Summary

Moves the library from Java 11 to Java 21. Build configuration and CI only —
no source changes were needed, the existing sources compile unchanged.

• pom.xml: java.version 11 → 21; maven.compiler.source/target replaced
  by maven.compiler.release, which also rejects references to APIs newer than
  the target
• pom.xml: pinned maven-compiler-plugin in pluginManagement (the
  version.compiler.plugin property existed but was wired to no plugin, so the
  version was floating); JaCoCo 0.8.10 → 0.8.12 for Java 21 bytecode
• CI: java-version: 21 in the three workflows, and distribution: adopt →
  temurin since adopt is deprecated and publishes no Java 21 builds
• README: Java version requirement updated in both places

Tested

mvn clean install on Temurin 21.0.12 — compiled at release 21, 99/99 tests
green, jar built.

japicmp could not be run locally: it fetched an EU Login HTML page instead of
the baseline jar through the corporate proxy, so the run used
-Djapicmp.skip=true. The API-compatibility check needs to pass in CI here.

Note for reviewers

This raises the bytecode level of a published artifact, so every consumer must
be on Java 21 before taking the release — a major-version change under semver.
Jira targets ECL 2.0.0 for that reason.

A stale origin/java21 branch (June 2025) reached the same pom conclusions
independently; it predates a year of develop and should probably be deleted.
It also bumps maven-source-plugin to 3.3.1, which this branch does not.

Jira: TEDEFO-5287