Update CHANGELOG.md (#65095)

* Add the Swift 5.8 release date.
* Format the note, to resemble a DocC aside.

Author: benrimmington

CHANGELOG.md

@@ -1,6 +1,7 @@
 # CHANGELOG
 
-_**Note:** This is in reverse chronological order, so newer entries are added to the top._
+> **Note**\
+> This is in reverse chronological order, so newer entries are added to the top.
 
 ## Swift 5.9
 
@@ -92,6 +93,8 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 
 ## Swift 5.8
 
+### 2023-03-30 (Xcode 14.3)
+
 * [SE-0376][]:
 
   The `@backDeployed(before:)` attribute may now be used to extend the availability of a function to OS releases prior to the introduction of that function as ABI.