Remove tailing comma in matrix JSON

Many tools are permissive about a trailing comma in a JSON array, but
not all. We use `act` to run GH actions locally and it is throwing an
error trying to parse this line.

Author: plemarquand

.github/workflows/swift_package_test.yml

@@ -9,7 +9,7 @@ on:
       macos_xcode_versions:
         type: string
         description: "Xcode version list (JSON)"
-        default: "[\"16.3\", \"16.4\", \"26.3\", \"26.4.b2\", ]"
+        default: "[\"16.3\", \"16.4\", \"26.3\", \"26.4.b2\"]"
       macos_exclude_xcode_versions:
         type: string
         description: "Exclude Xcode version list (JSON)"