Allow Symfony 6, require PHP 7.3+

[kbond]

This PR does the following:

• Allows Symfony 6
• Bump laminas/laminas-diagnostics min version to 1.9
• Require PHP 7.3+
• Drop sensiolabs/security-checker dev dep

Blocked By:

• https://github.com/enlightn/security-checker/pull/26

[kbond]

Merging for now - will need to fix the tests later.