Use GEF version 3.24

[lredor]

This allows to benefit of eclipse-gef/gef-classic#426 and the new corresponding APIs. These new APIs, will be used afterward in Sirius to clean code done for eclipse-sirius/sirius-desktop#359. Currently, the code corresponding to these new APIs has been duplicated no Sirius side (for example in PR 360).

Version GEF 3.20 will be probably released for Eclipse 2024.06.

[pcdavid]

As of now, the oldest target platform supported is Eclipse 2025-06, which includes GEF 3.24.
Any API available in GEF 3.24 and later are thus usable if it can help fix issues and/or reduce clutter in GMF.
I'll have to have a closer look to get an actual list.