docs: add Logger.warn deprecation as common mistake

smells/common-mistakes.md

@@ -1160,3 +1160,25 @@ end
 **Why it's OK here:** There's no conditional logic — every iteration tests the exact same behavior with a trivially predictable result. If one fails, the assertion message includes the specific value. The loop is purely for conciseness.
 
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+
+---
+
+## Logger.warn/2 is Deprecated
+
+**Smell:** Using `Logger.warn/2` instead of `Logger.warning/2`
+
+**Why it's wrong:** `Logger.warn/2` was deprecated in Elixir 1.11. The standard function is `Logger.warning/2`.
+
+**Example — broken:**
+```elixir
+Logger.warn("Connection lost: #{reason}")
+```
+
+**Example — fixed:**
+```elixir
+Logger.warning("Connection lost: #{reason}")
+```
+
+**Source:** [Elixir 1.11 Changelog](https://hexdocs.pm/elixir/1.11/changelog.html#logger-improvements)
+
+<!-- PATTERN_COMPLETE -->