refactor(handler): rename observers

Rename observers but with backward compatibility, relevant documentation

tests/test_dispatcher/test_deprecated.py

@@ -0,0 +1,49 @@
+import pytest
+
+from aiogram.dispatcher.event.observer import TelegramEventObserver
+from aiogram.dispatcher.router import Router
+
+OBSERVERS = {
+    "callback_query",
+    "channel_post",
+    "chosen_inline_result",
+    "edited_channel_post",
+    "edited_message",
+    "errors",
+    "inline_query",
+    "message",
+    "poll",
+    "poll_answer",
+    "pre_checkout_query",
+    "shipping_query",
+    "update",
+}
+
+
+def test_deprecated_handlers_name():
+    from aiogram import __version__
+
+    minor_partial = int(__version__.split(".")[1])
+
+    if minor_partial >= 2:
+        do_assert = pytest.raises(AttributeError)
+    else:
+        do_assert = pytest.warns(DeprecationWarning)
+
+    router = Router()
+
+    async def _(__):
+        ...
+
+    with do_assert:
+        for decor in OBSERVERS:
+            getattr(router, decor + "_handler")
+
+        assert all(
+            isinstance(getattr(router, handler + "_handler"), TelegramEventObserver)
+            for handler in OBSERVERS
+        )
+
+    assert all(
+        isinstance(getattr(router, handler), TelegramEventObserver) for handler in OBSERVERS
+    )