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

src/NadekoBot/Migrations/Mysql/20220429044757_bank.cs

@@ -0,0 +1,42 @@
+using System;
+using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace NadekoBot.Migrations.Mysql
+{
+    public partial class bank : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "bankusers",
+                columns: table => new
+                {
+                    id = table.Column<int>(type: "int", nullable: false)
+                        .Annotation("MySql:ValueGenerationStrategy", MySqlValueGenerationStrategy.IdentityColumn),
+                    userid = table.Column<ulong>(type: "bigint unsigned", nullable: false),
+                    balance = table.Column<long>(type: "bigint", nullable: false),
+                    dateadded = table.Column<DateTime>(type: "datetime(6)", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("pk_bankusers", x => x.id);
+                })
+                .Annotation("MySql:CharSet", "utf8mb4");
+
+            migrationBuilder.CreateIndex(
+                name: "ix_bankusers_userid",
+                table: "bankusers",
+                column: "userid",
+                unique: true);
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "bankusers");
+        }
+    }
+}