All ebuilds are updated to EAPI 8

[Anard]

OK. As you asked in previous PR, now all ebuilds are updated and I add an example script in generator folder to document the update. Script isn't executable as it's just example commands used for this update and need to be adapted for future changes.

Hope no mistakes will make some ebuilds unusable. :) Anyway thanks for merging, now eclean distfiles command don't show errors anymore :+1:

[stefan-langenmaier]

I had another look. So let's merge this and wait for feedback. :)

[Anard]

Nice, thx for your work.

[stefan-langenmaier]

Thanks for your patience and work!