[Infra] Use 18.3.1 simulator for Combine integration tests

[ncooke3]

Trying to fix the failures in the Combine nightlies. I've tried but have been unable to reproduce.

Update –– Was reproducing a crash locally. Traced it back to an assertion Combine API that was checking for errors. The 18.4 sim's networking is flaky and the pipeline assertion crashes the testing suite. Moving down to 18.3.1 sim for testing.

Fix #14787

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