Upgrade Etcd to v3.6.0

## Feature Request

### Describe your feature request related problem

Etcd v3.6.0 has been released. https://etcd.io/blog/2025/announcing-etcd-3.6/

Its performance has been significantly improved

> In this release, we reduced average memory consumption by at least 50%
> Compared to v3.5, etcd v3.6 delivers an average performance improvement of approximately 10% in both read and write throughput

In addition, this [feature](https://github.com/etcd-io/etcd/pull/14066) is helpful for PD. Currently, the middleware on the PD server side exists in each interface, which is very difficult to manage. The middleware implemented by PD itself cannot cover the interface of the etcd server.

### Describe the feature you'd like

Upgrade Etcd to v3.6.0

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy

As https://etcd.io/blog/2025/announcing-etcd-3.6/#upgrade-issue mentioned,

> Upgrade issue
This section highlights a common issue [issues/19557](https://github.com/etcd-io/etcd/issues/19557) in the etcd v3.5 to v3.6 upgrade that may cause the upgrade process to fail. For a complete upgrade guide, refer to [Upgrade etcd from v3.5 to v3.6](https://etcd.io/docs/v3.6/upgrades/upgrade_3_6/).
>
> The issue was introduced in etcd v3.5.1, and resolved in v3.5.20.
>
> Key takeaway: users are required to first upgrade to etcd v3.5.20 (or a higher patch version) before upgrading to etcd v3.6.0; otherwise, the upgrade may fail.

We need upgrade to etcd v3.5.20 firstly.