Disabled dropdown item

[vpicone]

Closes #4949

TODO: fix broken tests

Changelog

New

• Update Downshift to v4. This makes it so the disabled items are skipped when navigating via keyboard
• Styles for individual disabled options

Changed

• Update internal Downshift props to accomodate breaking changes

Testing / Reviewing

• We jumped 3 versions in Downshift. I got all the breaking changes but we should probably manually test the Downshift components: (ComboBox, Dropdown, FilterableMultiSelect, Multiselect)

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[joshblack]

Can we do the jump to v4 in a minor? Had assumed we couldn't since we forward props to downshift with the downshiftProps option 😞

[vpicone]

@joshblack gonna close this for now, maybe keep track of it if you wanna update Downshift in the future (it should prob be it's own PR that would be a blocker for this otherwise simple implementation. Could do a little man in the middle to guard folks passing in custom props that have been deprecated. They only deprecated the name, not any functionality.