Drop support for Plone 5.2 and Python 3.8

[ericof]

We should plan the next major release of plone.restapi (10.x) to drop support for Python 3.8, already end of life, and Plone 5.2 (will be end of life with the release of Plone 6.1.)

[mauritsvanrees]

5.2 is end of life since end of October. I have updated the release schedule yesterday.