Updated the version number and release date to be current release v#3.1.6

PiperOrigin-RevId: 639864767
Change-Id: I94a2c81fe125c99bdcfca07b557eebeadca542e9

Author: dhoke-megha

doc/changelog.rst

@@ -2,7 +2,7 @@
 Changelog
 =========
 
-Upcoming version (not yet released)
+Version 3.1.6 (Jun 3, 2024)
 -----------------------------------
 
 General