Refactor Burnup charts application to update Apollo cache in immutable way

As a step for migrating the codebase to Apollo Client 3.x, we need to ensure all the applications using Apollo update cache in an immutable way (see Breaking changes in Apollo 3.x).

There are a few places in burnup charts where we are mutating an Apollo cache. We need to set assumeImmutableResults to true on application's Apollo Client and fix all the errors caused by this.