Upgrade Gradle wrapper and GitHub actions versions

[sbesson]

In preparation of the next steps implementing the publication of the library to Maven Central, this PR makes routine build system updates:

• upgrades GitHub actions to use versions running on Node.js 24 - see https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/. ilammy/setup-nasm@v1 still has warnings but there has been no new version of this action running on Node.js 24
• upgrade Gradle wrapper version to 9.4.1 and update build.gradle accordingly. Java 17 is the minimal version for building the library while Java 11 is retained as the minimal runtime version in agreement with https://www.openmicroscopy.org/2026/03/24/end-of-java-8-support.html
• bump the Java version using in the GitHub workflow to use either 17 for building or 17, 21 and 25 for the test matrix