Replace /api/subscriptions/ REST calls with GraphQL queries

[Mark Moes]

The formfields use the /api/subscriptions/ endpoint with filters:

        subscriptionsWithFilters: build.query<
            NodeSubscription[],
            { filters: string }
        >({
            query: ({ filters }) => ({
                url: `${SUBSCRIPTIONS}/${filters}`,
                method: 'GET',
                headers: {
                    'Content-Type': 'application/json',
                },
            }),
            extraOptions: {
                baseQueryType: BaseQueryTypes.fetch,
            },
        }),

This endpoint will be removed in orchestrator-core 2.3.0 and should be replaced with a GraphQL query.