Update benchmark tool to same macOS version as DocC (#1124)

rdar://141185270

Author: d-ronnqvist

bin/benchmark/Package.swift

@@ -14,7 +14,7 @@ import PackageDescription
 let package = Package(
     name: "benchmark",
     platforms: [
-        .macOS(.v10_15)
+        .macOS(.v12)
     ],
     products: [
         .executable(