update immer to v5

[phryneas]

it might be better to let this rest for a week or so

Immer v5 was released today, with support for Map and Set.
While I believe it is usually adviced against using those in Redux, I also know that people do so anyways - those might benefit from this update.

On the other hand, this will bump the size of the immer dependency from gzipped 4.5kb to 6.3kb. ( https://bundlephobia.com/[email protected] ).

There is immerjs/immer#449 which aims to reduce bundle size again, and this is very new, so it might be a good idea to monitor immerjs/immer#354 if any other issues arise and decide on merging this in a week or two, so this PR is more of a reminder to check back later ;)

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[markerikson]

The biggest issue here is that our serialization check middleware will explicitly throw errors if it sees a Map or Set in the state or action. So, this doesn't really buy us anything useful.

I'm not opposed to updating eventually, but that selling point doesn't help here.

[phryneas]

Good point, hadn't thought of that! Guess we'll have no use at all from immer v5 at the moment.