bump min mac version to v12 (#938)

Author: dayaffe

Package.swift

@@ -23,7 +23,7 @@ let libXML2TargetOrNil: Target? = nil
 let package = Package(
     name: "smithy-swift",
     platforms: [
-        .macOS(.v10_15),
+        .macOS(.v12),
         .iOS(.v13),
         .tvOS(.v13),
         .watchOS(.v6)

smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/PackageManifestGenerator.kt

@@ -24,7 +24,7 @@ class PackageManifestGenerator(
                 writer.write("name: \$S,", ctx.settings.moduleName)
 
                 writer.openBlock("platforms: [", "],") {
-                    writer.write(".macOS(.v10_15), .iOS(.v13)")
+                    writer.write(".macOS(.v12), .iOS(.v13)")
                 }
 
                 writer.openBlock("products: [", "],") {

smithy-swift-codegen/src/test/kotlin/software/amazon/smithy/swift/codegen/manifestanddocs/PackageManifestGeneratorTests.kt

@@ -30,7 +30,7 @@ class PackageManifestGeneratorTests {
         assertNotNull(packageManifest)
         val expected = """
     platforms: [
-        .macOS(.v10_15), .iOS(.v13)
+        .macOS(.v12), .iOS(.v13)
     ],
 """
         packageManifest.shouldContain(expected)