Add changelog for 3.6

[ahrtr]

@pav-kv @erikgrinaker Please let me know if I miss anything.

[erikgrinaker]

@pav-kv made some other changes that we may want to list:

• https://github.com/etcd-io/etcd/pull/14624
• https://github.com/etcd-io/etcd/pull/14633
• https://github.com/etcd-io/raft/pull/32

[ahrtr]

thx for the reminder. Done.

Two of them were done before we split the raft package into a separate repository, and were added into etcd's changelog. I also need to remove from etcd's changelog, https://github.com/etcd-io/etcd/blob/main/CHANGELOG/CHANGELOG-3.6.md#package-raft