Updates the `Multiplexed` option to combine the output of stdout and stderr,
changing the previous behavior where stderr was preferred over stdout.
Now, both streams are properly combined into a single stream.

Additionally, this commit enhance and adjust existing test cases for
the `Multiplexed` option:

* Modifies `TestExecWithMultiplexedResponse` to ensure that stdout and
  stderr are on the same stream, testing the new behavior of the
  `Multiplexed` option.
* Refactors `TestExecWithNonMultiplexedResponse` to use `stdcopy.StdCopy`
  for ensuring that `io.Reader` contains a separated stdout and stderr.

* main:
  bug:Fix AMQPS url (testcontainers#2462)
  chore: more compose updates in comments
  chore: use "docker compose" (v2) instead of "docker-compose" (v1) (testcontainers#2464)
  chore(deps): bump github/codeql-action from 2.22.12 to 3.24.9 (testcontainers#2459)