fix(core): correctly gc query when suspense is used when query unmounts while it's still fetching (#8168)

* fix(core): correctly gc query when suspense is used when query unmounts while it's still fetching

the `isFetchingOptimistic` workaround was added to make sure some "thing" I don't remember anymore works in suspense; we added tests for that;

however, in the meantime, we added a minimum gcTime for suspense queries, which also solves that problem (tests still work); this makes the `isFetchingOptimistic` workaround unnecessary, and this workaround is actually what causes the unmount issue

* chore: try to stabilize a flaky test