Revert "Merge branch 'sh-update-grape-gem' into 'master'"

This reverts the upgrade of Grape in !27276 (merged) because nil values are not coerced properly in some cases. As mentioned in #215936 (comment 332421415), we should:

1. Wait for Grape v1.4 to include https://github.com/ruby-grape/grape/pull/2040
2. Change all our API Array[] types to have default: [] to avoid this confusion
3. Add more specs around nil values (labels in particular)