Convert std::execution functionality to use member functions instead of tag_invoke

[msimberg]

Later versions of stdexec support both tag_invoke and member functions, but the standard will use member functions only. We should try to migrate to use member functions only where it makes sense with reasonable backwards compatibility to the old C++17 implementation in pika.

The following CPOs and customizations can be translated quite simply:

• set_value_t (Change set_value CPO to use member functions instead of tag_invoke #1295)
• set_error_t
• set_stopped_t
• start_t
• connect_t
• schedule_t

Sender adaptor customization we currently still do with tag_invoke, but this is not an as straightforward translation. E.g. tag_invoke(bulk_t, thread_pool_scheduler, ...) would need to be translated to transform_sender(thread_pool_scheduler_domain, bulk_sender). We don't have domains implemented for schedulers yet, so we'd need that before tackling this.