Remove Plone 6.0 / Python 3.8 support

[stevepiercy]

Per https://plone.org/download/release-schedule

Limitations

[snip]
Frameworks including Python, Node.js®, and React may drop support for older versions, forcing us to drop support for these versions as well. For example, Plone 6.0 works on Python 3.8, but this Python version reaches end of life in October 2024. At that point, Plone 6 will drop support for Python 3.8.

[MohammadBinAftab]

I think this issue has been fixed, as both Plone 6.0 and Plone 6.1 are using newer Python versions than 3.8.
as per https://plone.org/download/release-schedule

[stevepiercy]

@MohammadBinAftab that obsolete text remains on plone.org and @mauritsvanrees needs to edit it.

Aside from that, a simple search of 3.8 of the repo shows all the potential work ahead. However it may be OK to leave it as is, as well. We need the maintainers of plone.api to make a decision about Python support.