Backport of docs: Consul on Kubernetes specific upgrade info into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #18230 to be assessed for backporting due to the inclusion of the label backport/1.15.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@im2nguyen
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: unable to process merge commit: "034d4626b2eaf11217b5cfdfbc735bd085869905", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

---

Description

These updates address version-specific upgrade information for Consul on Kubernetes. This information already existed within the docs, so these edits make this information more visible on pages related to upgrading the version of consul-k8s.

Links

• JIRA ticket

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

---

Overview of commits

• 9b02a4c - 3cf5e28 - ecec4c8 - edee177 - 034d462 - f42e9bf - ff2441d - 270c965

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR

[hc-github-team-consul-core]

🤔 This PR has changes in the website/ directory but does not have a type/docs-cherrypick label. If the changes are for the next version, this can be ignored. If they are updates to current docs, attach the label to auto cherrypick to the stable-website branch after merging.