For some reason a low version crashes Psalm.

This should make it easier to work with Composer from machines using
different versions of PHP than the lowest one we use in the CI.

Allowing us to use more advanced checks and have full generic support.

This also adds a baseline file, since we have new violations being
reported but don't want to address them right now.

$ composer update phpunit/phpunit --with-all-dependencies

See https://github.com/sebastianbergmann/phpunit/issues/3879

The new version contains some improvements in handling mocks (https://github.com/sebastianbergmann/phpunit/issues/3602) and will help identify some existing issues in DBAL 3.0 tests:

There were 2 warnings:

1) Doctrine\Tests\DBAL\Driver\OCI8\OCI8StatementTest::testExecute with data set #0 (array('test', null, 'value'))
Method bindValue may not return value of type boolean

2) Doctrine\Tests\DBAL\Driver\OCI8\OCI8StatementTest::testExecute with data set #1 (array(null, 'test', 'value'))
Method bindValue may not return value of type boolean

1. Got rid of hard-coded exception mocks in favor of PHPUnit-generated ones where possible.
2. Reworked AbstractDriverTest::testConvertsException() to use a data provider to make testing all types of exceptions independent.
3. Updated PHPUnit since the reworked code relies on sebastianbergmann/phpunit#3604.