fix: Use BODY_LIMIT when creating app

[jgoz]

In this PR:

• Bug fix (non-breaking change which fixes an issue)

The BODY_LIMIT env variable was being used in the content type parser, but not when creating the fastify app. This meant the practical body limit could not be raised past 50MB.

This PR uses the BODY_LIMIT var for both values.

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

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[jgoz]

@matteovivona Does this change make sense?