set Doctrine to use nest transactions with savepoints

[tcitworld]

Using nested transactions without savepoints is actually deprecated by Doctrine: https://www.doctrine-project.org/projects/doctrine-dbal/en/current/reference/transactions.html#transaction-nesting

Without savepoints, a nested transaction can be rollbacked but not handled properly in the "real" transaction, leading to the following error: Transaction commit failed because the transaction has been marked for rollback only.

Ref https://github.com/nextcloud/server/pull/36528#issuecomment-1639913965 (and possibly) https://github.com/nextcloud/server/issues/38902#issuecomment-1598075391

TODO

• [x] See how existing tests go
• [ ] Add a test with a example of nested transaction rollback

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)

[ChristophWurst]

:eyes: @nickvergessen

[nickvergessen]

https://www.doctrine-project.org/projects/doctrine-dbal/en/current/reference/transactions.html#transaction-nesting

Nesting transactions without savepoints is deprecated, but is the default behavior for backward compatibility reasons.

[nickvergessen]

Since we are bumping the major mostlikely with 28 anyway, I'm fine with queuing it for that release.

[ChristophWurst]

Since we are bumping the major mostlikely with 28 anyway, I'm fine with queuing it for that release

showtime?

[tcitworld]

Rebased