Updated version to 3.12.3 and updated CHANGES.txt. (#7580)

* Updated version to 3.12.3 and updated CHANGES.txt.

* Re-ran generate_descriptor_protos.sh and made it more parallel.
diff --git a/generate_descriptor_proto.sh b/generate_descriptor_proto.sh
index e533d05..dc03fee 100755
--- a/generate_descriptor_proto.sh
+++ b/generate_descriptor_proto.sh
@@ -62,7 +62,7 @@
     PROTOC=$BOOTSTRAP_PROTOC
     BOOTSTRAP_PROTOC=""
   else
-    make $@ protoc
+    make -j$(nproc) $@ protoc
     if test $? -ne 0; then
       echo "Failed to build protoc."
       exit 1