Update etcd to v3.5.x

[disksing]

Development Task

etcd v3.5.0 is the latest stable release version which is also go mod compatible version when used as a library. As there are other applications and libraries using pd (or pd client) as a library, we should upgrade etcd to v3.5.0 to make it more community-friendly.

There was an attempt before #3873. But we cannot merge it because upgrading the tidb-dashboard's etcd will break the PD of the release branches since the PDs of the different branches all depend on the release branch's tidb-dashboard.

Therefore, before upgrading etcd, we need to resolve the dependency between pd and tidb-dashboard. Specifically, create separate go mods for pd and pd-server, so that only pd-server will depend on tidb-dashboard and we can upgrade pd without upgrading tidb-dashboard.

[maoueh]

I would like to see how this can be pushed faster. Right now depending on tikv client-go is really a pain because for example, we are completely unable to perform a go mod tidy in our project due to problem with the infamous etcd library dependencies.

It would be more than welcome that this is done sooner than later. Willing to help if some hands are required.