build(deps): remove discoverable_entity_bundle_classes

As this module is no longer supported, remove it from the codebase and
update all references to it within the custom code - instead manually
wrapping nodes with the `Post` or `Talk` class, or returning it from a
Repository.

Fixes: #465

phpstan.neon

@@ -7,10 +7,7 @@ parameters:
     - *Test.php
     - *TestBase.php
   checkMissingIterableValueType: false
-  ignoreErrors:
-  - '#Call to an undefined method Drupal\\Core\\Field\\FieldItemListInterface::referencedEntities()#'
-  - '#Call to an undefined method Drupal\\views\\Plugin\\views\\query\\QueryPluginBase::addOrderBy().#'
-  - '#Call to an undefined method GuzzleHttp\\ClientInterface::post().#'
 includes:
   - vendor/mglaman/phpstan-drupal/extension.neon
   - vendor/phpstan/phpstan-deprecation-rules/rules.neon
+  - phpstan-baseline.neon