Switch to use Plone 6 in tests as default in acceptance testing, 5.x with essential testing

[sneridagh]

Also, update/modernize the convenience buildout included in Volto repo.

[davisagli]

@sneridagh What do you mean by "essential testing"?

[sneridagh]

@davisagli uh, I was under the assumption that I set up "essential" testing on 6 (a subset of the full suite, to improve a bit CI performance) but it seems that I refrained to do so at the last moment 😅.

Anyways, we could introduce it, so we start only to test a basic subset on Plone 5 from that moment on.

[davisagli]

This was done in #3700