Add test / documentation around SchemaUpgrader behavior

[trevor-scheer]

The release of 2.7 and more specifically join/v0.4 has been troublesome for multiple reasons. One assumption I made that hasn't helped the situation is that at least one subgraph would have to explicitly opt-in to fed 2.7 by updating the version in their federation @link directive.

Unfortunately, the schema upgrader upgrades subgraphs to the latest federation spec version by default when the subgraph is identified as a non-fed2 subgraph, i.e. no @link federation spec is provided.

The test added here demonstrates this behavior, but mostly serves as a required touchpoint for upgrading the federation spec version with some documentation describing the behavior and why it might cause challenges.

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