fix: Promote rack to runtime dependency because of 5.63.1

[kaiuhl]

Fixes #592

5.63.1 (specifically 9d69b6ef78c93e43e73e968685e3cd44342bd537) introduced a reliance on rack/media_type which was previously only a development dependency of this gem.

Checklist

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• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
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[philnash]

Including a rack middleware is only important when rack is part of the application. Rather than add another runtime dependency, we've changed this in #602 so that if Rack is not present, the middleware doesn't get loaded. Version 5.65.1 was just released with this update, so you can now install the latest version and it will work again.

[philnash]

Thank you for contributing though!