Fix Connection::prepare() return type

[2e3s]

I've checked doctrine/dbal since versions 2.6, none returns Driver\Statement.
https://github.com/doctrine/dbal/blob/v2.6.0/lib/Doctrine/DBAL/Connection.php#L856
https://github.com/doctrine/dbal/blob/3.6.x/src/Connection.php#L1044

[2e3s]

I'm not clear on failures because if run locally, the result is the same with or without the change. CI seems broken on the main branch?

[orklah]

Tests have been failing for some time, no need to worry :) maybe @weirdan will find time to look at this :)

Thanks!

[weirdan]

That would be after I get back to Psalm core (read: not very soon). Based on how it goes here I probably won't look into these test failures this year.