Fixes grpc#8037

It's preferred to setup dependency exclusions and declare the
problematic dependency as a direct dependency, because overwriting
the dependency (in this case org.apache.httpcomponents:httpcore),
while it may pass the checks in the build, does not have any
effect in the produced pom.xml after the library is published
to Maven Central.

Google-http-client-jackson2 depends on jackson-core.
Jackson-core depends on junit (with test scope).
If we don't exclude the junit dependency, :grpc-all:dependencies
(unexpectedly) fails to resolve dependencies. Details:
grpc#8078 (comment)