Combine milk and heavy cream in a large bowl. Add lime juice and whisk until thickened, about 1 minute. Spread 1 cup filling on bottom of deep-dish pie plate or 11-inch oval casserole. Top with single layer of Ritz crackers. Repeat, alternating layers of filling and crackers until pie plate is full (finish with a layer of filling). Sprinkle with grated lime zest. Refrigerate at least 2 hours or up to overnight. Serve.