Upgrade to RCP 2026-03 and latest Nebula / NatTable releases

Upgrade to RCP 2026-03 and latest Nebula / NatTable releases.

This means also upgrading to Java 21 in the build process at least, because RCP 2025-03 minimally requires Java 21. See #916 (closed) and !108.

P2 repository cloning tasks

• Clone 2026-03 release/update P2 repositories (once it gets released)
  • https://www.simantics.org/update/2026-03
  • https://www.simantics.org/update/eclipse/updates/4.39
  • Done 16.2.2026. Must be redone once 2026-03 is actually released
• Clone 2026-03 SimRel Orbit P2 repo
  • https://www.simantics.org/update/tools/orbit/simrel/orbit-aggregation/2026-03
  • Done 16.2.2026. Must be redone once 2026-03 is actually released
• Clone Nebula 3.2.0 P2 repo
  • https://www.simantics.org/update/nebula/releases/3.2.0/release
  • https://www.simantics.org/update/nebula/releases/3.2.0/incubation
• Clone Nattable 2.6.0 P2 repo
  • https://www.simantics.org/update/nebula/nattable/releases/2.6.0/repository

TODO tasks

• Package JRE 21 in https://www.simantics.org/download/jre
  • Must be manually done by admin
  • Find jre21-hotspot.zip symlink and jre21.0.10+7-hotspot.zip.
• Switch .tpd files to use the new P2 update sites:
  • https://www.simantics.org/update/2026-03
  • https://www.simantics.org/update/eclipse/updates/4.39
  • https://www.simantics.org/update/tools/orbit/simrel/orbit-aggregation/2026-03
  • https://www.simantics.org/update/nebula/releases/3.2.0/release
  • https://www.simantics.org/update/nebula/releases/3.2.0/incubation
  • https://www.simantics.org/update/nebula/nattable/releases/2.6.0/repository
• Upgrade all Simantics bundles to require Java 21 because Eclipse RCP since 2025-03 requires at least Java 21.
• Upgrade to Tycho 5.x (requires Java 21)
• Resolve any build and runtime problems that arise from the switch
  • Fix conflicts between Eclipse Orbit and Simantics external dependencies
  • Runtime problems are much harder to find
• Upgrade https://gitlab.simantics.org/simantics/build-ci-images branch jdk21 to package
  • ant 1.10.15
  • maven 3.9.9+ (required by Tycho 5.x) (3.9.12)
  • upgrade used Eclipse build that is embedded in the build container to Eclipse 4.38
    • See simantics/build-ci-images@49533aa121fc6ff03673d6a5b20a008f7fa7c5cd

Finalization tasks

• Revert issue #1098 workaround once the RCP is updated