Update Rack gems (major)

This MR contains the following updates:

Package	Update	Change
rack	major	'~> 2.2.6', '>= 2.2.6.2' -> '~> 3.0.4', '>= 3.0.4.2'
rack-cors	major	'~> 1.1.1' -> '~> 2.0.0'
rack-oauth2	major	'~> 1.21.3' -> '~> 2.2.0'

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