Add general info on upgrade policy to main upgrade doc

[jberkus]

To the master upgrade doc, add the following version-agnostic upgrade policies/advice for the Etcd subproject:

1. Only upgrades across one minor version (e.g. etcd 3.4 to 3.5) are supported. Upgrades that skip a minor version (e.g. etcd 3.4 to 3.6) are not supported and will likely fail.
2. Users should always update to the most recent patch version before upgrading minor versions.
3. Upgrades from EOL versions may work, but are no longer supported.

[neeraj542]

Hi @jberkus
I'd like to work on this, please assign issue to me. I’ll open a PR with the proposed docs change.

[ivanvc]

/assign @neeraj542