Set 'assumeImmutableResults' to false by default

Right now, until #35898 (closed) is not resolved, the freezeResults config has no actual effect. This means when we are mutating the Apollo cache we don't see UI changes even when the cache is updated and we have no console error to track the issue.

This has already resulted in a broken design deletion mechanism (we have no new version on UI after design deletion) on master and gitlab.com and it might lead to unpredictable side-effects.

I propose to disable assumeImmutableResults at least until apollo-cache-inmemory is updated and preferably until gitlab-org/frontend/rfcs#34 (closed) is resolved.

Let's make this opt-in until we can make sure our update hooks on mutations follow the immutability principles