Updated release version to 0.15.1

Makefile

build.yaml

package.json

package.xml

src/core/lib/surface/version.c

src/csharp/Grpc.Auth/project.json

src/csharp/Grpc.Core/VersionInfo.cs

src/csharp/Grpc.Core/project.json

src/csharp/Grpc.HealthCheck/project.json

src/csharp/build_packages.bat

src/node/tools/package.json

src/python/grpcio/grpc_version.py

src/ruby/lib/grpc/version.rb

src/ruby/tools/version.rb

tools/distrib/python/grpcio_tools/grpc_version.py

tools/distrib/python/grpcio_tools/setup.py

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/doxygen/Doxyfile.core

tools/doxygen/Doxyfile.core.internal