Update dependencies and fix tests.

[EreMaijala]

• These changes add support for PHP 8.0 and 8.1. Minimum PHP version is raised to 7.4.
• Switches from zend to laminas for up-to-date dependencies that support recent PHP versions.
• Removes unused dependencies (twig, service-manager).
• Makes curl tests more robust.
• Resurrects the testMtomMessage test and add laminas-mail dependency that laminas-mime needs for it to work.
• Adds return types and fixes other deprecation issues.

[EreMaijala]

I'm continuing the work with further refinements in our own fork.