Fails to compile because asio::execution::executor is unexpected in concept

Compiling cobalt via CMake gives me the following error message:

```
/tmp/boost-src/libs/cobalt/include/boost/cobalt/concepts.hpp(65,25): error: expected concept name with optional arguments
   65 |   {t.get_executor()} -> asio::execution::executor;
      |                         ^
1 error generated.
```