Update downshift

[tay1orjones]

We're two major versions behind on our downshift dependency. We attempted to update to v6 a few times before and hit major roadblocks - #9839, most recently #11331.

Primary drivers for this update include:

• Ensure we avoid any future CVEs related to outdated transient dependencies of v5.x
• There are features and fixes included in the update that will resolve some issues we have

Related issues blocked by downshift being outdated

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1. [a11y]: Dropdown typeahead needs to handle space key differently #10634
   adopter: external component: dropdown package: @carbon/react role: dev 🤖 severity: 2 type: a11y ♿ type: bug 🐛 version: 11 +8
   
2. Update downshift: research sprint #14102
   role: dev 🤖 type: infrastructure 🤖 +2
   

Something else related to this is an idea mentioned here, to expose our ListBox internals #6177 so consumers can directly use them with downshift. This could be part of the path to upgrading downshift and/or deprecating existing parts of our API. I'm not 100% sure, but it's something to consider and explore.

[francinelucca]

This will be a research issue first to gauge the possibility/size of this effort and we can re-discuss/prioritize and move from there