Update everything to JavaSE-21

[NiklasRentzCAU]

Under more recent versions of Eclipse, SPVizi-generated plugins have some problems, as Eclipse may overwrite the JavaSE-17 build requirements, breaking the build of the generated plugins. Updating to JavaSE-21 will probably resolve this issue. Furthermore, the Tycho version we currently use here does not seem to support JavaSE-21 yet, so an update here will also be necessary.