Upgrade IntelliJ platform to 2025.1

Oostveen, K. (Koen, Student B-AM,B-TCS)requested to merge
koen/intellijml:platform-2025 into master

Does the obvious, will make it run on the newest versions of IntelliJ. Tested on 2025.1 with gradlew runIde and on 2025.2 manually.

  • Updated IntelliJ platform to 2025.1-2025.2
  • Updated Java to 21 because IntelliJ would complain otherwise
  • Updated Kotlin plugin because it would break otherwise
  • Updated Gradle because... you know the deal
  • Bumped PsiViewer dependency
  • Bumped version to 1.1.9

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