Bump version number to 1.6.0 (#1314)
Bump version number to 1.6.0 (#1314)

Motivation:

We plan on tagging a release soon.

Modifications:

- Bump the version to 1.6.0

Result:

The version in the default user-agent string will match the released
version.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift