Update docs for Plone 6.1 release

[stevepiercy]

• Verify that Cookieplone uses Plone 6.1 final.
• Update Cookieplone output wherever it is documented.
• With buildout- or pip-based installs, either replace all instances of https://dist.plone.org/release/6-latest/ with https://dist.plone.org/release/6.1-latest/ or verify that the former redirects to the latter.
• In docs/install/create-project-classic-ui.md, update guilabels and workflow as needed.
• Update docs/admin-guide/install-buildout.md and docs/admin-guide/install-pip.md to use Python versions for Plone 6.1.
• Consider new replacement keys for PLONE_BACKEND_MINOR_VERSION and PLONE_BACKEND_PATCH_VERSION.
• [docs] Version policy needs update for Plone 6.1, Volto 18, Node.js compat volto#6540
• Split SUPPORTED_PYTHON_VERSIONS replacement when 6.1 is released #1674

Originally posted by @davisagli in #1714 (comment)

[davisagli]

With buildout- or pip-based installs, replace all instances of https://dist.plone.org/release/6-latest/ with https://dist.plone.org/release/6.1-latest/

I assume that https://dist.plone.org/release/6-latest/ will redirect to the latest 6.1 release once 6.1 is final

[stevepiercy]

@davisagli thanks! Updated item. Feel free to update the description with other items as they arise.