Add general info on upgrade policy to main upgrade doc

[neeraj542]

Add version-agnostic upgrade policies/advice for the Etcd subproject:

• Only upgrades across one minor version are supported (e.g. 3.4 to 3.5)
• Users should update to the most recent patch version before upgrading minor versions
• Upgrades from EOL versions may work but are no longer supported

Fixes DONE

Fixes #1114

[k8s-ci-robot]

Hi @neeraj542. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ivanvc]

/ok-to-test

[ivanvc]

@neeraj542, thanks for you pull request :) Could you please address the linter warnings? Thanks

[jberkus]

@ronaldngounou @wendy-ha18 can one of you do a quick readability review on this?

[neeraj542]

thanks for guiding on my contribution, I've fixed the linter issues. Please review this PR.

cc: @ivanvc , @jberkus

[ronaldngounou]

Thank you @neeraj542 for your contribution.
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: neeraj542, ronaldngounou
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[neeraj542]

my pleasure @ronaldngounou, still its showing me as Merging Blocked, could anyone pls look into this..

[ronaldngounou]

@jberkus for approval