Description of php-jwt library import into Moodle

Dependencies
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- The lib/lti1p3 library currently depends on version 6.0 of php-jwt.
- There are usages of this library in mod/lti too. Please check these.

Instructions
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1.  Check dependencies to confirm suitability of the new version of the library (see above).
2.  Visit [https://github.com/firebase/php-jwt].
3.  Click on 'X releases'.
4.  Download the latest release.
5.  Unzip it in lib as php-jwt.
6.  Update entry for this library in lib/thirdpartylibs.xml.
