Upgrade grape dependency

Adds support for Grape 2.0 and Rack 3.

Changes in Grape 2 don't affect the routing helpers we have:

• https://my.diffend.io/gems/grape/1.7.1/2.0.0
• https://github.com/ruby-grape/grape/blob/v2.0.0/CHANGELOG.md

Related to gitlab-org/gitlab#396273