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>

extensions/embed/less/forum.less

@@ -50,7 +50,7 @@
       padding: 15px 15px;
 
       .scrolled & {
-        .box-shadow(0 2px 6px var(--shadow-color));
+        box-shadow: 0 2px 6px var(--shadow-color);
       }
     }