Upgrade firebase/php-jwt and adjust implementation

[julien-nc]

• use latest firebase/php-jwt
• bump min NC version to 24 to drop php 7.3 support to be able to use recent firebase/php-jwt
• adjust parameters to jwt::decode

[julien-nc]

I don't know where the php7.3 check come from 😁

[juliusknorr]

Still a requirement from branch protection, we can remove that then :)

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