[3.x] Bot API 6.0 (#890)

* Base implementation

* Bump license

* Revert re-generated tests

* Fix tests, improved docs

* Remove TODO

* Removed unreachable code

* Changed type of `last_synchronization_error_date`

* Fixed wrongly cleaned code

docs/api/methods/get_chat_menu_button.rst

@@ -0,0 +1,44 @@
+#################
+getChatMenuButton
+#################
+
+Returns: :obj:`MenuButton`
+
+.. automodule:: aiogram.methods.get_chat_menu_button
+    :members:
+    :member-order: bysource
+    :undoc-members: True
+
+
+Usage
+=====
+
+As bot method
+-------------
+
+.. code-block::
+
+    result: MenuButton = await bot.get_chat_menu_button(...)
+
+
+Method as object
+----------------
+
+Imports:
+
+- :code:`from aiogram.methods.get_chat_menu_button import GetChatMenuButton`
+- alias: :code:`from aiogram.methods import GetChatMenuButton`
+
+In handlers with current bot
+----------------------------
+
+.. code-block:: python
+
+    result: MenuButton = await GetChatMenuButton(...)
+
+With specific bot
+~~~~~~~~~~~~~~~~~
+
+.. code-block:: python
+
+    result: MenuButton = await bot(GetChatMenuButton(...))