build: update dependency firebase-functions to v6

[angular-robot]

This PR contains the following updates:

Package	Type	Update	Change
firebase-functions	devDependencies	major	^5.0.0 -> ^6.0.0

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[!WARNING] Some dependencies could not be looked up. Check the Dependency Dashboard for more information.

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Release Notes

firebase/firebase-functions (firebase-functions)

### [`v6.0.0`](https://redirect.github.com/firebase/firebase-functions/releases/tag/v6.0.0) [Compare Source](https://redirect.github.com/firebase/firebase-functions/compare/v5.1.1...v6.0.0) - **Breaking:** Change default entrypoint of the firebase-functions package to v2 instead of v1 ([#​1594](https://redirect.github.com/firebase/firebase-functions/issues/1594)) - Add [@​deprecated](https://redirect.github.com/deprecated) annotation on functions.config() API ([#​1604](https://redirect.github.com/firebase/firebase-functions/issues/1604))

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