Specify the platform version as the fixtures depend on swift concurrency (#9318)

Specify the platform version as the fixtures depend on swift
concurrency.

### Motivation:

Tests failing locally due to code in the fixtures depending on Swift
Concurrency features.

### Modifications:

Specify the platform version in the fixtures.

### Result:

Tests pass

Author: rconnell9

Fixtures/SwiftMigrate/ExistentialAnyWithCommonPluginDependencyMigration/Package.swift

@@ -4,6 +4,9 @@ import PackageDescription
 
 let package = Package(
     name: "ExistentialAnyMigration",
+    platforms: [
+        .macOS(.v10_15)
+    ],
     targets: [
         .target(name: "Library", dependencies: ["CommonLibrary"], plugins: [.plugin(name: "Plugin")]),
         .plugin(name: "Plugin", capability: .buildTool, dependencies: ["Tool"]),

Fixtures/SwiftMigrate/ExistentialAnyWithPluginMigration/Package.swift

@@ -4,6 +4,9 @@ import PackageDescription
 
 let package = Package(
     name: "ExistentialAnyMigration",
+    platforms: [
+        .macOS(.v10_15)
+    ],
     targets: [
         .target(name: "Library", plugins: [.plugin(name: "Plugin")]),
         .plugin(name: "Plugin", capability: .buildTool, dependencies: ["Tool"]),