fix(taxonomy tree): options remain selected when switching in single mode

This is a hack fix since the tree is already built on top of React anti-patterns, which cause this in the first place. In the future a full refactor is really required to sort this out at its roots.

By adding an unstable `key`, it forces the component to re-render on every render. Without this, React assumes nothing has changed and does not react to the `selectedKey` update.

For large trees with everything expanded, this could cause performance issues but should be okay in most cases.

Author: Jake Stanger

src/controls/modernTaxonomyPicker/taxonomyTree/TaxonomyTree.tsx

@@ -659,6 +659,7 @@ export function TaxonomyTree(
           className={styles.taxonomyItemFocusZone}
         >
           <ChoiceGroup
+            key={new Date().getTime()}
             options={options}
             selectedKey={
               props.selection && props.selection.getSelection()[0]?.id