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

src/NadekoBot/Migrations/PostgreSql/20220623090729_stondel-db-cache.cs

@@ -0,0 +1,37 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+#nullable disable
+
+namespace NadekoBot.Migrations.PostgreSql
+{
+    public partial class stondeldbcache : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "streamonlinemessages",
+                columns: table => new
+                {
+                    id = table.Column<int>(type: "integer", nullable: false)
+                        .Annotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.IdentityByDefaultColumn),
+                    channelid = table.Column<decimal>(type: "numeric(20,0)", nullable: false),
+                    messageid = table.Column<decimal>(type: "numeric(20,0)", nullable: false),
+                    type = table.Column<int>(type: "integer", nullable: false),
+                    name = table.Column<string>(type: "text", nullable: true),
+                    dateadded = table.Column<DateTime>(type: "timestamp with time zone", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("pk_streamonlinemessages", x => x.id);
+                });
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "streamonlinemessages");
+        }
+    }
+}