update change log for v3.6 to include details on deprecated flags.

[Niket1997]

CHANGELOG: add decommissioned flags

To update the changelog with the deprecated flags that re decommissioned in v3.6.

Issue: 19492

Reference: This file in v3.5 contains the list of deprecated flags, also there is a comment against flags that are deprecated & decommissioned in v3.6.

[k8s-ci-robot]

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[k8s-ci-robot]

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[ahrtr]

All the changes happened before we release v3.6.0-rc.0, so please update the v3.6.0-rc.0 changelog.

• Please add an item "Removed v2 http proxy" under the section "Deprecations" something like below,

- Removed the [v2 http proxy](https://github.com/etcd-io/etcd/pull/14034). Note that the following flags have already been removed,
  - `--proxy`
  - `--proxy-failure-wait`
  - `--proxy-refresh-interval`
  - `--proxy-dial-timeout`
  - `--proxy-write-timeout`
  - `--proxy-read-timeout`

• Also add an item "Deprecated v2store" under the section "Deprecations" something like below,

- Deprecated [v2store, v3store is the sole source of truth for the membership data](https://github.com/etcd-io/etcd/issues/12913). Note that the following flags have already been removed, 
  - `--enable-v2`
  - `--experimental-enable-v2v3`

[ahrtr]

To double confirm: have you confirmed that this is a complete list?