refactor: cloud_firestore - remove deprecated embedding v1 & upgrade dependencies

[juliansteenbakker]

Description

This PR:

• Removes reference to deprecated embedding v1
• Upgrades gradle & dependencies
• Upgrades Android compile SDK version
• Applies some code improvements

Related Issues

As described in #7127 (comment)
part of #7138

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

[russellwheatley]

LGTM