Gemfile: drop sorbet environment

[vidusheeamoli]

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We're now aiming on introducing developers to Sorbet, hence it seems like a good time to abandon ENV["HOMEBREW_SORBET"] in our Gemfile and introduce Sorbet.
cc - @issyl0 @mistydemeo

[issyl0]

This feels like a nice milestone!

[jonchang]

Can this be unversioned like the rest of Gemfile, or are patch versions incompatible with each other?

[issyl0]

@jonchang This was pinned in #8055 because newer versions surface over 100 new typing errors in Tapioca's autogenerated RBI files which we can't do anything about. It's been a while now and Sorbet is on 0.5.5866, so I've done some investigation into how we can unpin this and get back to a sensible number of errors that are fixable. TL;DR on my draft PR: When Tapioca ships a new version, we'll be able to unpin this.

[MikeMcQuaid]

@jonchang @issyl0 @vidusheeamoli What we should do is remove the version pinning from Gemfile and have something running in CI that ensures we don't merge upgrades that break things. If that's going to be more than a few days: we should add a comment to the Gemfile explaining why the pins is in place and what we're waiting on.