Updated CHANGES.txt and package.xml with changes for 3.15.7

Author: acozzette

CHANGES.txt

@@ -1,3 +1,12 @@
+2021-04-02 version 3.15.7 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
+
+  C++
+  * Remove the ::pb namespace (alias) (#8423)
+
+  Ruby
+  * Fix unbounded memory growth for Ruby <2.7 (#8429)
+  * Fixed message equality in cases where the message type is different (#8434)
+
 2021-03-10 version 3.15.6 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
 
   Ruby

php/ext/google/protobuf/package.xml

@@ -22,7 +22,7 @@
  </stability>
  <license uri="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</license>
  <notes>
- No new changes in 3.15.6
+ No new changes in 3.15.7
  </notes>
  <contents>
   <dir baseinstalldir="/" name="/">