Change the collation for the hashtags table

Changes the collation for the hashtags table to utf8mb4_unicode_520_ci
when the driver is MySQL or MariaDB. Otherwise the default collation of
utf8mb4_unicode_ci is used which conflates all characters outside of the
basic multilingual plane. This meant that any hashtag that is using a
script not in the BMP ends up matching with any other hashtag with a
script outside the BMP if it has the same length.

database/migrations/2025_07_31_164635_change_hashtags_collation.php

@@ -0,0 +1,36 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        if (config('database.default') === 'pgsql')
+            return;
+
+        Schema::table('hashtags', function (Blueprint $table) {
+            $table->string('name')->collation('utf8mb4_unicode_520_ci')->change();
+            $table->string('slug')->collation('utf8mb4_unicode_520_ci')->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        if (config('database.default') === 'pgsql')
+            return;
+
+        Schema::table('hashtags', function (Blueprint $table) {
+            $table->string('name')->change();
+            $table->string('slug')->change();
+        });
+    }
+};