Version bump to 1.6

BUILD

CMakeLists.txt

Makefile

build.yaml

doc/g_stands_for.md

gRPC-Core.podspec

gRPC-ProtoRPC.podspec

gRPC-RxLibrary.podspec

gRPC.podspec

package.json

package.xml

src/core/lib/surface/version.c

src/cpp/common/version_cc.cc

src/csharp/Grpc.Core/Version.csproj.include

src/csharp/Grpc.Core/VersionInfo.cs

src/csharp/build_packages_dotnetcli.bat

src/csharp/build_packages_dotnetcli.sh

src/node/health_check/package.json

src/node/tools/package.json

src/objective-c/!ProtoCompiler-gRPCPlugin.podspec