Update ci.yaml in flutter/cocoon to use either macOS 15.5 or 15.7.1

.ci.yaml

diff --git a/.ci.yaml b/.ci.yaml
index 112fb65..058f33f 100644
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -15,11 +15,11 @@
       device_type: "none"
   mac:
     properties:
-      os: "Mac-14|Mac-15"
+      os: "Mac-15.5|Mac-15.7"
       cpu: x86
   mac_arm64:
     properties:
-      os: "Mac-14|Mac-15"
+      os: "Mac-15.5|Mac-15.7"
       cpu: arm64
   windows:
     properties: