Remove JDK 21 workflow after Loom Build Tests implementation

[Copilot]

Summary

Removes the maven.yml workflow that uses JDK 21, as it's no longer needed after the introduction of the Loom Build Tests workflow in #27.

Background

The project requires Java 26 features and uses Project Loom's virtual threads. After #27 added the loom-tests.yml workflow that uses the latest Loom JDK from shipilev.net, maintaining a separate JDK 21 workflow became redundant.

Changes

• Removed .github/workflows/maven.yml (Java CI with Maven / build workflow using JDK 21)
• Kept .github/workflows/loom-tests.yml (Loom Build Tests / test-with-loom workflow)

Rationale

• The Loom Build Tests workflow provides more recent JDK features required by this project (Java 26)
• Single workflow reduces maintenance overhead and CI runtime
• The Loom build from shipilev.net includes all necessary features for building and testing this project

Closes #28

Original prompt

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This section details on the original issue you should resolve

<issue_title>Don't run Java CI with Maven / build (push) with JDK 21 anymore</issue_title>
<issue_description>After #27 there is no need for running the JDK 21 workflow on master, but just Loom Build Tests / test-with-loom (push) .
Remove the JDK 21 related workflows.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #29

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