Support PHP 8 and drop support for php 7.3? #19

Our pdsinterop/solid-auth package uses lcobucci/jwt and is used in both the Nextcloud and Standalone PHP Solid Servers.

When we started implementing Solid in PHP, in early/mid 2020, the lowest supported PHP version was 7.3 and the Nextcloud version was 20. Starting 2022, PHP 7.3 will no longer be supported (see https://www.php.net/supported-versions) and PHP 8.1 will have been released. As of last month Nextcloud 20 is also no longer supported.

The current Nextcloud releases support the following PHP versions:

✅ = supported
✔ = recommended

We need to start supporting PHP 8, however, there is a problem related to lcobucci/jwt, specifically, the PHP versions supported by that library. Version 3 supports PHP7 but not 8, version 4 supports PHP 8 but only 7.4. (not 7.3).

This means that, if we continue to use that package, we can support either PHP 7.3 or PHP 8, but not both.

My suggestion would be to drop support from PHP 7.3 so we can update all three of our libraries to lcobucci/jwt to v4 and start supporting PHP 8.

Companion issues have been opened in the Nextcloud and Standalone PHP repos for this issue:

• Support PHP 8 and drop support for php 7.3 php-solid-server#56
• Drop Nextcloud 20 (Add support for PHP8, drop support for PHP7.3) solid-nextcloud#59