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.
• [x] https://github.com/plone/volto/issues/6540
• [x] https://github.com/plone/documentation/issues/1674

Originally posted by @davisagli in https://github.com/plone/documentation/pull/1714#discussion_r1779697046

[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.