Narrow down driver return types

[derrabus]

Q	A
Type	improvement
BC Break	no
Fixed issues	N/A

Summary

On PHP 7.4, we can make use of covariance to be more specific about the return types of Driver, Connection and Statement classes.

[morozov]

This is a good point. We should be able to apply this approach in some more cases:

dbal/tests/Platforms/AbstractPlatformTestCase.php

Lines 38 to 41 in c8ddc4a

	/**
	* @return T
	*/
	abstract public function createPlatform(): AbstractPlatform;