Update Firebase PHP-JWT to ^6.1

[steveneaston]

Description

Updates Firebase PHP-JWT to the latest version (6.1.0) and adds the now required $alg argument for JWT::encode. Also updates the TestHelper to use Firebase\JWT\Key for JWT::decode.

Breaking changes outlined in the 6.0.0 release notes: https://github.com/firebase/php-jwt/releases/tag/v6.0.0.

Motivation and Context

6.0.0 fixes a security flaw
firebase/php-jwt#351
https://security.snyk.io/vuln/SNYK-PHP-FIREBASEPHPJWT-2434829

^6.0 is also now required by https://github.com/laravel/passport from ≥10.4.0.

How Has This Been Tested?

Existing tests make use of JWT::encode and JWT::decode and pass.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #297 (61da830) into main (1493c01) will not change coverage.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
src/OpenTok/Util/Client.php	75.9% <100.0%> (ø)

[steveneaston]

Merged in #318