Add mixed-cluster tests for X-Pack

[ywelsch]

We need tests that check that mixed-version X-Pack clusters are working correctly, in particular now that they're becoming the default distribution. Having such tests can uncover X-Pack BWC issues such as found in #30859.

[elasticmachine]

Pinging @elastic/es-core-infra

[gwbrown]

To resolve confusion for anyone who, like me, saw the rolling upgrade x-pack tests and thought this might have been addressed already, per @jakelandis (thanks!):

There is a difference between "mixed cluster" in a rolling upgrade and a "mixed cluster" test. The former (rolling upgrade) only tests the explicit things that are written for the rolling upgrade, and the latter (mixed cluster test) runs all of the YAML REST tests. We have rolling upgrade tests for both OSS and x-pack, but we only have "mixed cluster" tests that run the REST tests for OSS. We do not run the x-pack YAML REST tests in a mixed cluster as far as I know.