bump v1.2.x branch version to 1.2.1-pre2

CMakeLists.txt

Makefile

build.yaml

gRPC-Core.podspec

gRPC-ProtoRPC.podspec

gRPC-RxLibrary.podspec

gRPC.podspec

package.json

package.xml

src/cpp/common/version_cc.cc

src/csharp/Grpc.Auth/project.json

src/csharp/Grpc.Core.Testing/project.json

src/csharp/Grpc.Core/VersionInfo.cs

src/csharp/Grpc.Core/project.json

src/csharp/Grpc.HealthCheck/project.json

src/csharp/Grpc.Reflection/project.json

src/csharp/build_packages_dotnetcli.bat

src/csharp/build_packages_dotnetcli.sh

src/node/health_check/package.json

src/node/tools/package.json