Change MetadataGroup.group variable to HashSet

[solth]

Fixes #5543

This fixes one part of #4821

[matthias-ronge]

Could you replace HashSet with TreeSet? This would then also fix #4357. Just a question. You can also first merge this

[solth]

I considered using TreeSet instead of HashSet (as you already suggested during our conversation) but ultimately decided against it because of potential performance implications (see point 2.3 on https://www.baeldung.com/java-hashset-vs-treeset).

Although I can see the advantage of always having the metadata in the same order, currently I think avoiding any impacts on performance especially in the metadata editor has to have higher priority.