feat: upgrade github.com/benbjohnson/immutable

[mhilton]

Newer versions of this package were changed to use generics, making them incompatible with the current code. Update to the newest version. This has the nice side effect of reducing the amount of type assertions.

Closes #5477

Checklist

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[appletreeisyellow]

Thank you for taking care of this @mhilton!