Split `SUPPORTED_PYTHON_VERSIONS` replacement when 6.1 is released

[stevepiercy]

Replace the replacement SUPPORTED_PYTHON_VERSIONS with Plone-specific versions when 6.1 is released.

• PLONE_61_SUPPORTED_PYTHON_VERSIONS
• PLONE_60_SUPPORTED_PYTHON_VERSIONS

Originally posted by @stevepiercy in #1671 (comment)

[stevepiercy]

Done in:

• https://github.com/plone/documentation/blob/6.0/docs/conf.py#L318-L319
• https://github.com/plone/documentation/blob/6.0/docs/_inc/_install-python-plone60.md
• https://github.com/plone/documentation/blob/6.0/docs/_inc/_install-python-plone61.md