update generated code (#423)

Author: async-aws-bot

src/Enum/Runtime.php

@@ -7,6 +7,7 @@ class Runtime
     public const DOTNETCORE_1_0 = 'dotnetcore1.0';
     public const DOTNETCORE_2_0 = 'dotnetcore2.0';
     public const DOTNETCORE_2_1 = 'dotnetcore2.1';
+    public const DOTNETCORE_3_1 = 'dotnetcore3.1';
     public const GO_1_X = 'go1.x';
     public const JAVA_11 = 'java11';
     public const JAVA_8 = 'java8';
@@ -31,6 +32,7 @@ public static function exists(string $value): bool
             self::DOTNETCORE_1_0 => true,
             self::DOTNETCORE_2_0 => true,
             self::DOTNETCORE_2_1 => true,
+            self::DOTNETCORE_3_1 => true,
             self::GO_1_X => true,
             self::JAVA_11 => true,
             self::JAVA_8 => true,