[Doctrine Bridge] simplify session handler by using main connection
minor #10743 [Doctrine Bridge] simplify session handler (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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[Doctrine Bridge] simplify session handler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no because the driver connection could not be used anyway before
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

Use main connection as discussed in #10720

Commits
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6518b74 [Doctrine Bridge] simplify session handler by using main connection