Upgrade build requirement and fix application packaging

[sbesson]

Fixes #493

This fixes the packaging of the OMERO.insight application for OS X and Windows and upgrade.

Proposed backwards incompatible build changes:

• the minimal Java build requirement of the appliction is bumped to 17 LTS
• the minimal Java build requirement of the packages is bumped to 21 LTS for the package (due to the usage of -app-content option of jpackage - see https://docs.oracle.com/en/java/javase/21/docs/specs/man/jpackage.html)
• the OMERO.importer package is no longer built

Other changes:

• the Gradle wrapper is upgraded to 9.3.1
• the Gradle configuration files are upgraded to work with Gradle 9 and fix Gradle 10 warning
• the org.beryx.runtime Gradle plugin is used for packaging the application on Windows/OSX using jpackage and a runtime JRE
• buildSrc is removed
• various resources (configuration, icons) are moved under src
• the GitHub workflows are updated to use ./gradlew build and ./gradlew jpackage on the appropriate platforms

Remaining todos:

• add configuration to build the omero_ij uber-JAR
• fix the configuration loading for the packages and run commands