local task plugins discovery changed to use YamlDiscovery instead of annotated class discovery

After working with the initial implementation added in #2004334: Separate Tabs (MENU_LOCAL_TASK) from hook_menu(), the discovery mechanism for local tasks was changed to use the YAML discovery mechanism that was recently introduced in #2065571: Add YAML Plugin discovery

Local Task plugins are a good candidate for this YAML discovery since the vast majority of the time when module developers are adding one it will use the same, default, base class.

The original change notice at https://drupal.org/node/2044515 is updated to reflect this change in discovery.