If the Mariners are going to contend in the American League West in 2024, they’ll do so on the strength of their starting pitching. Through all the upheaval and uncertainty this winter, the Mariners approach the start of spring training next week having accomplished one of their primary objectives of the offseason: They made five notable trades to reshape their lineup and augment their bullpen without having to give up a prized starting pitcher. “And that’s really important to us,” general manager Justin Hollander said recently. For good reason. Boasting one of MLB’s best pitching rotations in…