Update module firebase.google.com/go to v3.13.0

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
firebase.google.com/go	require	minor	v3.12.1+incompatible -> v3.13.0

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Release Notes

firebase/firebase-admin-go

v3.13.0

Compare Source

New Features

• feat(auth): Exposing CustomToken() on auth.TenantClient (#​371)

Miscellaneous

• Deletes users slowly during the integration tests. (#​375)
• [chore] Release 3.13.0 (#​374)
• Add bulk get/delete methods (#​325)
• chore: Pointing CI badge to the dev branch (#​369)
• chore: Added go1.14 to the build matrix (#​368)
• chore: Automating the release publish step (#​366)

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