Improve class inheritance error messages (#7307)

Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Convert IMAPConnection to IMAP\Connection

IMAP: Declare `IMAPConnection` class as `final` (stub+test)

Updates the `IMAPConnection` class stub to make sure it has the `final` flag, and adds a test to verify it.