Move away from "Monocle" in favor of "Headless"

[tresf]

Switches to the new experimental glass.platform="Headless" property, removing the special monocle JavaFX dependency.

• This is built atop the jdk21 branch, but the only relevant commits should be those on or after d87055b.
• At time of writing this, this flag does not yet exist in OpenJFX as it's still undergoing internal R&D

Fixes #1253.

[tresf]

This has progressed significantly since, quoting:

[Gluon has] a working build in our public Nexus repository, that you can test:

<repository>
   <id>Gluon Releases</id>
   <url>https://nexus.gluonhq.com/nexus/content/repositories/releases/</url>
</repository>

<dependency>
   <groupId>org.openjfx</groupId>
   <artifactId>javafx-controls</artifactId>
   <version>24-headless+0-2024-12-02-101029</version>
</dependency>

In order to select the headless platform, you need to add:

-Dglass.platform=Headless 

This can run on sw pipeline as well (convenient for CI).

This PR depends on the switch to JDK21, making it a good candidate to rebased and merged into #1222 (edit: perhaps #1216) however this will depend on whether or not the repository is compatible with Ivy, so this PR was created as a stop-gap for testing interim headless builds. I'll do some integration testing.