[Howto]: Configure different PHP version for separate projects

### Have you already checked elsewhere?

- [X] I have read the [Devilbox documentation](https://devilbox.readthedocs.io/en/latest/)
- [X] I have checked the [Devilbox Troubleshooting Guide](https://devilbox.readthedocs.io/en/latest/)
- [X] I have checked the [Devilbox FAQ](https://devilbox.readthedocs.io/en/latest/support/faq.html)
- [X] I have checked the [Devilbox Howto](https://devilbox.readthedocs.io/en/latest/support/howto.html)
- [ ] I have checked the [Devilbox Forum](https://devilbox.discourse.group/)
- [ ] I have checked [existing issues](https://github.com/cytopia/devilbox/issues)
- [X] I have googled already with no luck
- [ ] I have not done any of the above

### What are you struggling with?

I am working on getting different versions of PHP running with different projects.

The error I am getting is the following when attempting to access a project that uses other than the default version of PHP:
```
Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
```

I am on an M1 Mac with:
Devilbox v3.0.0-beta-0.3 (2023-01-02)

I have run through the configuration instructions on https://github.com/cytopia/devilbox/pull/942

docker-compose.override.yml is active
PHP56 and PHP81 are loaded. PHP81 is the system default
backend.cfg consists of: conf:phpfpm:tcp:php56.local:9000

The Devilbox virtual hosts page displays the following in the backend column: tcp://php56.local:9000

Any suggestions on how to get it working would be much appreciated.

### What have you tried already?

Run through available documentation, validate configurations. Validate proper containers are running. Shelling into running containers.

### What is your goal?

Use different versions of PHP on different projects.