Conferences: remove past conferences, add upcoming ones Change-Id: I19b52546e7c5e0542b4fd76a248e1c4545df7e27 GitHub-Last-Rev: 36229b7ce11fa87f05b5678b9a1e9482e16862d9 GitHub-Pull-Request: golang/wiki#85 Reviewed-on: https://go-review.googlesource.com/c/wiki/+/742640 Reviewed-by: Sean Liao <sean@liao.dev>