refactor(gatsby): update internal-data-bridge plugin to use createContentDigest

[smurrayatwork]

This updates gatsby's internal-data-bridge plugin to use createContentDigest, as requested by #8805.

First time gatsby contributor here! I was unsure as to how to add tests to this particular plugin, as I didn't see any for the other internal plugins either.

[pieh]

Tests for this are tricky, especially one for updating Site nodes. I don't love it, but let's merge You PR as-is. I verified it works locally.

[pieh]

Thanks @smurrayatwork!

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