Replace MultiJson with JSON in JSON middleware

[jKatt]

This PR removes MultiJson from the JSON middleware.
With the adjustments made to JSON a while ago, there is no need to use another gem for JSON parsing.

[johannesluedke]

ears is also used outside of our org. If someone relied on multi json and f.e. some custom oj config -- maybe we should make this a major version change

[johannesluedke]

to release it, don't forget to adjust version.rb and CHANGELOG

[jKatt]

ears is also used outside of our org. If someone relied on multi json and f.e. some custom oj config -- maybe we should make this a major version change

I thought about making it a minor change, as we are still on the 0 major version.
Bumping it to major 1 would mean we are finally on a production-ready major version 🤔

[johannesluedke]

ears is also used outside of our org. If someone relied on multi json and f.e. some custom oj config -- maybe we should make this a major version change

I thought about making it a minor change, as we are still on the 0 major version. Bumping it to major 1 would mean we are finally on a production-ready major version 🤔

ah, then minor is fine 👍