Fix message for __aenter__ in PLC2801 (#9492)

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## Summary

Fix the message for `__aenter__ ` in PLC2801 (introduced in
https://github.com/astral-sh/ruff/pull/9166)
There is no `aenter` builtin in Python, so the current message is
misleading.
I take the message from original lint
https://github.com/pylint-dev/pylint/blob/main/pylint/constants.py#L211

P.S. I think here should be more accurate synchronization with original
lint (e.g. the current implementation will not lint `__enter__` on my
first sight), but it is out-of-scope of this change.

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## Test Plan

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crates/ruff_linter/src/rules/pylint/rules/unnecessary_dunder_call.rs

@@ -273,7 +273,7 @@ impl DunderReplacement {
             "__str__" => Some(Self::Builtin("str", "Use `str()` builtin")),
             "__subclasscheck__" => Some(Self::Builtin("issubclass", "Use `issubclass()` builtin")),
 
-            "__aenter__" => Some(Self::MessageOnly("Use `aenter()` builtin")),
+            "__aenter__" => Some(Self::MessageOnly("Invoke context manager directly")),
             "__ceil__" => Some(Self::MessageOnly("Use `math.ceil()` function")),
             "__copy__" => Some(Self::MessageOnly("Use `copy.copy()` function")),
             "__deepcopy__" => Some(Self::MessageOnly("Use `copy.deepcopy()` function")),