chore(main): release 6.8.0

[release-please[bot]]

:robot: I have created a release beep boop

6.8.0 (2023-06-14)

Features

• add support for P-384 curve (#515) (5de4323)

Bug Fixes

• handle invalid http responses (#508) (91c39c7)

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[release-please[bot]]

:robot: Release is at https://github.com/firebase/php-jwt/releases/tag/v6.8.0 :sunflower: