Add deprecation-notice to rule CA2109 (#31797)

* Add deprecation notice (see dotnet/roslyn-analyzers#5974)

* Update docs/fundamentals/code-analysis/quality-rules/ca2109.md

Co-authored-by: Genevieve Warren <[email protected]>

* Update docs/fundamentals/code-analysis/quality-rules/ca2109.md

Co-authored-by: Genevieve Warren <[email protected]>

Co-authored-by: Genevieve Warren <[email protected]>

Author: x789

docs/fundamentals/code-analysis/quality-rules/ca2109.md

@@ -24,6 +24,11 @@ ms.author: gewarren
 
 A public or protected event-handling method was detected.
 
+> [!NOTE]
+> This rule has been deprecated. It last shipped with Microsoft.CodeAnalysis.Analyzers v3.3.0.
+> 
+> The rule was removed because the threat that the analyzer warned about (an untrusted intermediary hooking a privileged event handler to a privileged event invoker) hasn't existed since .NET Framework 4.5.
+
 ## Rule description
 
 An externally visible event-handling method presents a security issue that requires review.