📖 Improve version support documentation

[fabriziopandini]

What this PR does / why we need it:
Improving version support matrix to better surface the practices we are following.
TL;DR; we try to go over and behind Kubernetes support matrix to meet the users where they are, but at the same time there should be boundaries to what we can reasonably support/validate with our CI signal.

Which issue(s) this PR fixes:
Fixes #4444

/cc @CecileRobertMichon @vincepri @enxebre @sbueringer

[killianmuldoon]

/lgtm

[vincepri]

/approve

[sbueringer]

Sounds great.
/lgtm

/hold
@fabriziopandini in case you want to squash before merge

[stmcginnis]

/lgtm

[enxebre]

/lgtm

[vincepri]

/hold cancel
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vincepri

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [vincepri]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[fabriziopandini]

/cherry-pick release-1.1
given that this can help the users to better understand our support matrix

[k8s-infra-cherrypick-robot]

@fabriziopandini: new pull request created: #6175

In response to this:

/cherry-pick release-1.1
given that this can help the users to better understand our support matrix

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.