A Study to Evaluate the Adverse Events, Efficacy, and Optimal Dose of Intravenous (IV) ABBV-400 in Combination With IV Fluorouracil, Leucovorin, and Budigalimab in Adult Participants With Locally Advanced Unresectable or Metastatic Gastric, Gastroesophageal Junction, or Esophageal Adenocarcinoma