v4: Fix use of use_gateway

[kalzoo]

Currently, use_gateway is accepted as an argument to both:

• QcsClient::load
• Qpu.new

but it is not passed between the two in the load() call. A user could get past this by constructing a QcsClient themselves, but this is not ideal.

Two ways to fix this:

1. Pass the argument to QcsClient::load, or:
2. Better yet, make the changes as described in Standardize use_gateway as part of configuration qcs-sdk-rust#310 and then remove the use_gateway argument here, which is inconsistent with the other aspects of client configuration.

[MarquessV]

closed via #1598