Bump to eclipse 2023-12

[dvojtise]

Motivation

As every year, a new version of Eclipse is available on December https://projects.eclipse.org/releases

We need to update the studio to benefit from its new features.

• update target platforms to Eclipse 2023-12
• update splash screen
• update tycho version
• update xtend/xtext version
• update impacted components to support current version of xtext/xtend (TODO / TO BE REFINED)

I also hope this will have effect on #307

[dvojtise]

The new 2023-12 has removed the old org.eclipse.xtext.generator that is replaced by org.eclipse.xtext.xtext.generator and need several upgrades in the .mwe2

It is not possible to get an older version simultaneously to the new one.

We need to upgrade all our xtext .mwe2 (including Timesquare, Melange, ...)