downgrade acme version to avoid x509 deprecation

[markdboyd]

Changes proposed in this pull request:

We need to stay on acme < 2.8 for now because acme 2.8.0 uses pyopenssl 23.3.0 which deprecates x509, but the acme package hasn't been updated to handle that deprecation.

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

There are no apparent security issues with acme < 2.8.0 at the moment. And will upgrade acme as soon as the upstream issue with the pyopenssl dependency is resolved.