[feat] Increase valid version range for lxml dependency

[CalebCourier]

Description
Increase valid version range for lxml dependency

Why is this needed
When building for slim linux platforms, older versions (<5.x) of lxml only have prebuilt wheels for python 3.11 and below.

Implementation details
Upgrade to lxml 5.x and test. If everything still works as expected, increase range to allow 5.x in pyproject.toml

End result
End users can use a larger version range of lxml with guardrails-ai.

Bonus points if we can make lxml an optional dependency on a rail extra, but this would require a rather large refactor.