Updated version to 3.15.8 (#8463)

* Update protobuf version

* Updated CHANGES.txt and package.xml for 3.15.8

Protobuf-C++.podspec

diff --git a/Protobuf-C++.podspec b/Protobuf-C++.podspec
index 26e3825..13adedd 100644
--- a/Protobuf-C++.podspec
+++ b/Protobuf-C++.podspec
@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name     = 'Protobuf-C++'
-  s.version  = '3.15.7'
+  s.version  = '3.15.8'
   s.summary  = 'Protocol Buffers v3 runtime library for C++.'
   s.homepage = 'https://github.com/google/protobuf'
   s.license  = '3-Clause BSD License'