Provide PHP 8.1 support

[weierophinney]

This patch provides PHP 8.1 support, via the following changes:

• Updates the PHP constraint to add ~8.1.0
• Updates the way the package replaces zend-mime
  • Replaces the "replace" section of the package with "conflict", and conflicts with all versions of zend-mime
  • Removes the dependency on laminas-zendframework-bridge
• Updates to and applies laminas-coding-standard 2.2 rules
• Updates testing to inline the Headers class

We have a circular dependency scenario between laminas-mail and laminas-mime. laminas-mail depends on laminas-mime for its MIME parsing capabilities. laminas-mime decodes headers to a laminas-mail Headers instance.

Unfortunately the Headers class implements Iterator, and, as such, we get deprecations under 8.1 unless the #[ReturnTypeWillChange] attribute to affected methods.

The solution here is to temporarily inline the Headers class as a test asset until laminas-mail is updated to work on PHP 8.1.