Switch remaining ml tests to new style Requests

In elastic#29623 we added `Request` object flavored requests to the low level
REST client and in elastic#30315 we deprecated the old `performRequest`s. This
changes all calls in the `x-pack/plugin/ml/qa/native-multi-node-tests`,
`x-pack/plugin/ml/qa/single-node-tests` projects to use the new
versions.

Author: nik9000