Update min kubernetes version in the install doc

[AlanGreene]

Changes

tektoncd/website#614
and https://tektoncd.slack.com/archives/CJ62C1555/p1720961833907299

v0.61.0 updated the minimum version of Kubernetes to 1.28.

Update the docs to reflect this so the website has the correct content and avoid confusing users.

/kind documentation

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
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Release Notes

NONE

[chitrangpatel]

/lgtm

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[AlanGreene]

/cherry-pick release-v0.61.x

[tekton-robot]

@AlanGreene: new pull request created: #8123

In response to this:

/cherry-pick release-v0.61.x

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