Upgrade Grape from 0.15.0 to 0.18.0

What does this MR do?

Upgrades Grape from 0.15.0 to 0.18.0. This removes rack-mount which is an unmaintained gem.

• Changelog: https://github.com/ruby-grape/grape/blob/2d34f29b52827157d3abf43a569f205783709212/CHANGELOG.md#0180-1072016
• UPGRADING.md: https://github.com/ruby-grape/grape/blob/5683154c28f6c78ad7f3035061aca7a34ddefa06/UPGRADING.md#upgrading-to--0170

Are there points in the code the reviewer needs to double check?

Tests need to pass.

Why was this MR needed?

rack-mount is unmaintained and doesn't support Rack 2.0.

What are the relevant issue numbers?

Working toward #14286 (moved). Fixes #18226 (closed).