Updated CHANGES.txt
Updated CHANGES.txt

Now that 3.13.0 has been released, this commit updates CHANGES.txt to
mention 3.13.0 instead of 3.13.0-rc1 and to include the recent PHP
changes. I also added a short explanation of the 3.12.4 release.

Fixes #7820.

google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf