Revert "Use memory-optimized CI runners"

This happened to work once, but has failed every time since. Unclear what is going on, but switching to the memory-optimized job runners has resulted in wiremock related tests failing. Increasing the timeout all the way to 1s has no effect, suggesting we aren't cpu limited in some way. Requires investigation into how exactly wiremock is providing the expected failures to the test suite. Feels a little like the test suite is making a real network connection and a firewall is dropping the packets, but seems unlikely.

This reverts commit 6980aae7