Deprecate the Active-Set shmem_sync()

[kholland-intel]

While deprecating the active-set collectives in #1017, it became clear that shmem_sync() cannot be easily deprecated because of the C11 generic here. This generic can select the shmem_sync function that we are trying to add the deprecated attribute to, which leads to a deprecated warning on the team-based C11 shmem_sync() as well.