chore: handle deprecations from 1.x (#3909)

* chore: drop deprecated `post_number_index` column

* chore: remove deprecated `FlagsWillBeDeleted` event

* chore: `Migration::addSettings` can still be needed

* chore: `settings->get` default can still be needed

* chore: deprecated `$default` in `Settings::serializeToForum` extender

* chore: deprecated request `actor` attribute

* chore: already handled

* chore: remove `RecompileFrontendAssets::whenSettingsSaved`

* chore: remove `getReadIds`

* chore: `Model::dateAttribute` extender

* chore: `evented` js util

* chore: `WelcomeHero` js hidden prop

* chore: attributes pushData with relations

* chore: app request options `extract`

* chore: itemlist deprecations

* chore: `search` state

* chore: `getMentionText`

* chore: deprecated non-registered store type

* chore: `Button` title

* chore: `Modal` deprecations

* chore

* chore: deprecated `less` mixins

* Apply fixes from StyleCI

* fix

* fix: phpstan

* fix

* fix

---------

Co-authored-by: StyleCI Bot <bot@styleci.io>

framework/core/migrations/2023_10_23_000000_drop_post_number_index_column_from_discussions_table.php

@@ -0,0 +1,12 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+
+return Migration::dropColumns('discussions', ['post_number_index' => '']);