Support netstandard 2.0 and netstandard 2.1 for mysql storage

src/DotNetCore.CAP.MySql/DotNetCore.CAP.MySql.csproj

 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
     <AssemblyName>DotNetCore.CAP.MySql</AssemblyName>
     <PackageTags>$(PackageTags);MySQL</PackageTags>
   </PropertyGroup>

test/DotNetCore.CAP.MySql.Test/ConnectionUtil.cs

@@ -38,7 +38,7 @@ namespace DotNetCore.CAP.MySql.Test
 
         public static MySqlConnection CreateConnection(string connectionString = null)
         {
-            connectionString = connectionString ?? GetConnectionString();
+            connectionString ??= GetConnectionString();
             var connection = new MySqlConnection(connectionString);
             connection.Open();
             return connection;

test/DotNetCore.CAP.MySql.Test/DatabaseTestHost.cs

@@ -34,7 +34,7 @@ namespace DotNetCore.CAP.MySql.Test
                 var storage = GetService<IStorageInitializer>();
                 var token = new CancellationTokenSource().Token;
                 CreateDatabase();
-                storage.InitializeAsync(token);
+                storage.InitializeAsync(token).GetAwaiter().GetResult();
                 _sqlObjectInstalled = true;
             }
         }