Replace \PDO::FETCH_* constants to indicate fetch mode with an enumeration

For the change record and the questions on the MR.

Follow up: 📌 [PP-1] Introduce a StatementBase abstract class Postponed , to create a common base class for statement objects.

The CI pipeline is not happy.

Juggled a bit with typehints.

All the code change good to me.
I have updated the CR and the IS.
The CI pipeline is green for all by core supported databases.
All my questions have been answered.
For me it is RTBC.

Thanks for review @daffie!

I'm still trying to do some tuning with the typehints in PdoTrait - to make them as accurate and PHPStan-proof as possible. It may turn out that CodeSniffer will be unhappy, in which case I think we should silence its failures.

Changes to return typehints were OK for CodeSniffer, good - but now this needs review again I suppose, sorry @daffie

Stricter typing is highlighting a bug. On that.

Reviewable again.

All looks good to me.
Back to RTBC.