Revert "Remove all EF migrations"

This reverts commit bb5b9458e8.

osu.Game/Migrations/20210514062639_AddAuthorIdToBeatmapMetadata.cs

@@ -0,0 +1,23 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+namespace osu.Game.Migrations
+{
+    public partial class AddAuthorIdToBeatmapMetadata : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<int>(
+                name: "AuthorID",
+                table: "BeatmapMetadata",
+                nullable: false,
+                defaultValue: 0);
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "AuthorID",
+                table: "BeatmapMetadata");
+        }
+    }
+}