[CHANGELOG-3.6] List all the removed flags

[ahrtr]

What would you like to be added?

We need to update changelog-3.6 to list all the flags (i.e. --enable-v2), which are available in release-3.5, but already removed in release-3.6.

We also need to clearly document this as tracked in etcd-io/website#926 (specifically the upgrade checklist etcd-io/website#961)

cc. @ivanvc @jmhbnz

Why is this needed?

To guarantee a smooth upgrade from 3.5 to 3.6.

[ivanvc]

I honestly don't think I have the capacity to take any more tasks right now. If James is unavailable, I think we should try recruiting someone to help with this.

[ahrtr]

I think this should be part of the upgrade guide (etcd-io/website#963). Users should check whether they have configured any of the flags (which has already been removed in 3.6) before upgrading to 3.6.

cc @Zanda256 @shivamgcodes

[Niket1997]

/assign