chore: require firebase/php-jwt v6 to force fix for key/algorithm type confusion

[alperendurmus]

Possibility of Reintroducing HS256/RSA256 Type Confusion (CVE-2021-46743)
firebase/php-jwt#351
GHSA-8xf4-w7qw-pjjw

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[bshaffer]

@alperendurmus Thank you for your contribution. Please sign the CLA!

[bshaffer]

Looks like we need to update the base version of a few other dependencies

[bshaffer]

We will have to drop support for PHP 5.6 and 7.0 before we are able to make this change.

[bshaffer]

Done in #2431