DBAL 3.x removes PDO dependency

[javiereguiluz]

Main deprecations:

• The support of PDO::PARAM_*, PDO::FETCH_*, PDO::CASE_* and PDO::PARAM_INPUT_OUTPUT constants in the DBAL API is removed.
• \Doctrine\DBAL\Driver\PDOStatement does not extend \PDOStatement anymore.

See doctrine/dbal#2958

[stof]

Even the part meant to be shipped in the next 2.x are not there yet, so it is too early to rewrite the doc to use the new PARAM and FETCH constants rather than the PDO ones (we might even want to wait until these constants are part of a released 2.x version)

[javiereguiluz]

Closing in favor of the meta-issue #9401.