codemeta: Add GSI as "maintainer"

[ChristianTackeGSI]

GSI is providing resources for maintaining FairMQ. So let's document this in codemeta.json.

• Switch maintainer to schema.org version codemeta/codemeta#250 (comment)
  Codemeta < 4.0 does not allow anything but Persons in the maintainer field. I personally think, we should just ignore this.

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Checklist:

• Rebased against dev branch
• My name is in the resp. CONTRIBUTORS/AUTHORS file
• Followed the seven rules of great commit messages

[ChristianTackeGSI]

I had thought, that this would conflict with #478. But that's not true. So the only problematic point is Codemeta 4.0 being needed officially (which is not even released), and OSSR sticking at 2.0. But as I already mentioned above, I don't think, we should care too much.

[ChristianTackeGSI]

Having looked more at it, the sponsor field could also be an option. Pro: It already allows "Organization"s today.

To be honest, I prefer the maintainer field for FairMQ.