CONTRIBUTING: update Slack join and channel link
CONTRIBUTING: update Slack join and channel link

    Slack channel was renamed from #libbpf-go to #ebpf-go, so let's
    update CONTRIBUTING.md accordingly.

Signed-off-by: Fabio Falzoi <af21799f430c5704ed4f25b5c44c77a7db1c59b4@gmail.com>

cilium/ebpf,cilium/ebpf