Cygwin: Add pthread_key fix in release note

Signed-off-by: Takashi Yano <[email protected]>

Author: tyan0

winsup/cygwin/release/3.6.1

@@ -45,3 +45,7 @@ Fixes:
 - Fix up cached DOS attributes when trying to create the same file in
   two (or more) threads/processes concurrently.
   Addresses: https://cygwin.com/pipermail/cygwin/2025-April/257871.html
+
+- Fix deadlock when calling pthread_key_create in the destructor of
+  a pthread_key.
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257705.html