Update dependency firebase-admin to v8

[renovate-bot]

This PR contains the following updates:

Package	Type	Update	Change
firebase-admin (source)	dependencies	major	7.4.0 -> 8.2.0

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Release Notes

firebase/firebase-admin-node

v8.2.0

Compare Source

• [fixed] Gracefully handling array-like objects in messaging.sendAll() and
  messaging.sendMulticast() APIs.
• [fixed] Updated the metadata server URL (used by the application default credentials)
  to the v1 endpoint.

v8.1.0

Compare Source

• [added] admin.projectManagement().listAppMetadata() method to list the app summary of up to 100
  apps in a Firebase project
• [added] admin.projectManagement().setDisplayName() method to update the display name of a
  Firebase project
• [fixed] The SDK now automatically retries HTTP calls failing due to 503 errors.

v8.0.0

Compare Source

• [changed] Dropped support for Node 6. Developers must use Node 8.13.0 or
  higher.
• [changed] Upgraded Cloud Firestore client to v2.0.0.

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