Get Phpstan job to pass with DBAL 3

In order not to disrupt contributors, we made sure it would not fail the build although it really does not pass at all:

Line 52 in e09f126

continue-on-error: "${{ matrix.status == 'experimental' }}"

If you want to help us fix it or get closer to fixing it, you can reproduce the failures locally by running (from the 2.10.x branch)

composer require doctrine/dbal ^3.0
vendor/bin/phpstan

When this issue is fixed, we should create a new one about making this job no longer experimental.

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