Set ChangeLog version and release date

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/ChangeLog b/ChangeLog
index 3231cb4..087d22e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS x.x.x branch released xxxx-xx-xx
+= mbed TLS 2.27.0 branch released 2021-07-07
 
 API changes
    * Update AEAD output size macros to bring them in line with the PSA Crypto