MAINTAINERS: move labeled networking to "supported"

My current employer, Microsoft, cares very much about the development
and maintenance of the upstream Linux Kernel so we can consider
labeled networking to be "supported" according to the definition in
MAINTAINERS:

  S: *Status*, one of the following:
      Supported:  Someone is actually paid to look after this.
      Maintained: Someone actually looks after it.
      ...

Signed-off-by: Paul Moore <paul@paul-moore.com>