chore: Update unstructured-client

[Coniferish]

The pinned version of unstructured-client was changed from >=0.15.1 to <=0.18.0 because more recent versions of the client broke some functionality. This needs to be changed back and any relevant updates need to be made.

In addition to fixing any errors that come from updating the client, there are some newer sdk parameters that will be supported. test_unstructured_ingest/unit/pipeline/reformat/test_chunking.py and unstructured/ingest/pipeline/reformat/chunking.py will need to be updated to reflect this.

Additional fixes:

• A new release of pikepdf has been made since feat: update dependencies and remove constraint on pydantic #2841 and lxml<5 can be removed from the constraints.