fix: unique key cannot be created with TEXT column

See:

issue: https://discuss.flarum.org/d/18316-flarum-seo/587

65535 supported by mysql and pgsql (for 2.x)

migrations/2023_02_01_create_seo_meta_table.php

@@ -11,7 +11,7 @@
 
                 // Object information
                 $table->integer('object_id');
-                $table->text('object_type');
+                $table->string('object_type', 65535);
 
                 // Make the combination Object ID and Object Type unique
                 $table->unique(['object_id', 'object_type']);