Update minor version to 7.3.0 (#20815)

Co-authored-by: Mmbot <mmbot@mattermost>

model/version.go

@@ -13,6 +13,7 @@ import (
 // It should be maintained in chronological order with most current
 // release at the front of the list.
 var versions = []string{
+	"7.3.0",
 	"7.2.0",
 	"7.1.0",
 	"7.0.0",