Gemfile: set sorbet version

[vidusheeamoli]

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The new Sorbet release introduced 100+ new errors which we'll be dealing with after we have more files transitioned to true in sorbet/files.yaml. Hence, this PR sets Sorbet to the version that we're working on currently.

[vidusheeamoli]

cc @issyl0 @mistydemeo

[issyl0]

Great, thanks!

[issyl0]

Worth noting that the new errors Sorbet brings are in Tapioca's autogenerated gem RBI files, so I'm not sure there's much we can do. I've been keeping an eye out for new Tapioca and Sorbet versions, but 🤷.