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 (Make set_error use member function instead of tag_invoke #1369)
• set_stopped_t
• start_t (Change operation state start CPO to use member functions #1392)
• connect_t (Change sender connect CPO to use member functions #1393)
• schedule_t
• get_env_t (Change get_env CPO to use member functions #1394)

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.