Updated CHANGES.txt and PHP changelog for 3.15.6

Author: acozzette

CHANGES.txt

@@ -1,3 +1,8 @@
+2021-03-10 version 3.15.6 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
+
+  Ruby
+  * Fixed bug in string comparison logic (#8386)
+
 2021-03-04 version 3.15.5 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
 
   Ruby

php/ext/google/protobuf/package.xml

@@ -22,8 +22,7 @@
  </stability>
  <license uri="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</license>
  <notes>
- New changes in 3.15.5:
- * Fixed quadratic memory use in array append (#8379)
+ No new changes in 3.15.6
  </notes>
  <contents>
   <dir baseinstalldir="/" name="/">