Flatten source and test directory structure

Removed the `Dbal` prefix from the class names and moved them to the `Doctrine\Tests\DBAL` namespace.

Made some platform class names reflect the lowest server version they support

The `PostgreSqlPlatform`, `SQLAnywherePlatform` and `SQLServerPlatform` classes have been renamed to `PostgreSQL94Platform`, `SQLAnywhere16Platform` and `SQLServer2012Platform` respectively.

When deprecating an old platform version, we can deprecate and later remove a specific class. Otherwise, there's no way to express the deprecation on the class level and the upgrade notes contain sentences like: "`XYZPlatform` and `XYZKeywords` now represent XYZ (version)".

Backport removal of legacy platforms and drivers from master

Deprecate the usage of the legacy platforms and drivers

Upgrade PHPUnit to 9.0

Manually merge 2.11.x into 3.0.x

Drop support for PHP 7.2

Bump version to 2.11.0-DEV

Updated documentation for QueryBuilder::execute() return value type