WB-1671: Dropdown: use `combobox` role in all openers #4931

Job	Run time
Prime node_modules cache for primary configuration (ubuntu-latest, 20.x)	17s
Check for .changeset entries for all changed files (ubuntu-latest, 20.x)	56s
Publish npm snapshot (ubuntu-latest, 20.x)	1m 11s
Check build sizes (ubuntu-latest, 20.x)	1m 28s
Lint / Lint (ubuntu-latest, 20.x)	1m 6s
Test / Test (ubuntu-latest, 20.x, 1/2)	1m 5s
Test / Test (ubuntu-latest, 20.x, 2/2)	1m 18s
	7m 21s

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