Support C++20, replace implicit capture of this with explicit captures

[daantimmer]

C++20 deprecated the implicit capture of this for lambda captures.

This PR replaces all implicit captures by value [=], with their explicit equivalents.

[andreasbuhr]

Thanks for your contribution. The CI reports some problems, could you have a look at them please?

[daantimmer]

I will
@andreasbuhr done, missed two captures :-)