Update dependency firebase-functions to v2.3.1

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
firebase-functions	2.1.0 -> 2.3.1

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Release Notes

firebase/firebase-functions

v2.3.1

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• Adds region support for europe-west2 and asia-east2

v2.3.0

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• Adds pubsub.schedule()

v2.2.1

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• Adds functions.app() api to access the app instance used by functions
• improved types of the Change class to describe both before and after fields as non-optional
• Improve type of express.Request to include rawBody

v2.2.0

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• support for multiple regions on functions by passing extra region strings to functions.region()
• validation and typing improvements

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