Run tests on macOS 15.5 or 15.7 (#10336)

In preparation of upgrading our fleet to macOS 15.7, allow tests to run on either macOS 15.5 or 15.7.

*List which issues are fixed by this PR. You must list at least one issue.*
Fixes flutter/flutter#176931

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Author: okorohelijah

.ci.yaml

@@ -95,7 +95,7 @@ platform_properties:
         [
           {"dependency": "ruby", "version": "ruby_3.1-pod_1.13"}
         ]
-      os: Mac-14|Mac-15.5
+      os: Mac-15.5|Mac-15.7
       device_type: none
       cpu: arm64
       $flutter/osx_sdk : >-
@@ -108,7 +108,7 @@ platform_properties:
         [
           {"dependency": "ruby", "version": "ruby_3.1-pod_1.13"}
         ]
-      os: Mac-14|Mac-15.5
+      os: Mac-15.5|Mac-15.7
       device_type: none
       cpu: x86
       $flutter/osx_sdk : >-