Set `assumeImmutableResults` to true by default [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR is one of the steps to prepare for Apollo Client 3.x migration. We set assumeImmutableResults to true by default. Currently, it is done with a feature flag apollo_default_immutable enabled by default. This way we are able to turn it off if the change causes a production incident.

Does this MR meet the acceptance criteria?

Conformity

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