MDL-58040 blocks: add upgrade.txt note

blocks/upgrade.txt

 This files describes API changes in /blocks/* - activity modules,
 information provided here is intended especially for developers.
 
+=== 3.3 ===
+
+* block_manager::get_required_by_theme_block_types() is no longer static.
+
 === 3.1 ===
 
 * The collapsed class was removed from the navigation block to make it compatible with aria.