Fixed a bug with postgresql and mysql migration folders as well as compilation warning related to it

src/NadekoBot/Migrations/Mysql/20220428044612_stondel.cs

@@ -0,0 +1,26 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace NadekoBot.Migrations.Mysql
+{
+    public partial class stondel : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<bool>(
+                name: "deletestreamonlinemessage",
+                table: "guildconfigs",
+                type: "tinyint(1)",
+                nullable: false,
+                defaultValue: false);
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "deletestreamonlinemessage",
+                table: "guildconfigs");
+        }
+    }
+}