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 https://github.com/qzind/tray/commit/d87055beb13b35f0d8241800d577e9801a8a81bc.
• At time of writing this, this flag does not yet exist in OpenJFX as it's still undergoing internal R&D

Fixes https://github.com/qzind/tray/issues/1253.