Use work items API for requirements management

Before we deprecate&remove requirements API (#329432), we need to update UI to create/update/delete requirements (and requirement test reports) through issues/work item API (as requirement becomes just another type of issue / work item). This is blocked by having requirement widget support in API though (related to #323790 (closed))

function	query	issue
get requirements (index)	project_requirements.query.graphql	#382265
counts	project_requirements_count.query.graphql	#382469
create requirement	create_requirement.mutation.graphql	#382470
update requirement	update_requirement.mutation.graphql	#382471
destroy requirement	can't do that so 🤷	n/a
archive requirement	update_requirement.mutation.graphql	#382472

See https://docs.gitlab.com/ee/user/project/requirements/ and under the directory ee/app/assets/javascripts/requirements/queries where all these live.