Revert "Get rid of the legacy mongodb integration"

[rschu1ze]

#63279 introduced a new mongoDB driver. This was first released in v24.10. The old driver remained the default.

#63279 removed the old driver (appears in v24.12).

Many users only upgrade from LTS release to LTS release, but there has been no LTS release where old + new mongoDB drivers are available.

This PR reverts #63279 but makes the new driver the default. This will increase the margin of safety for users with LTS requirements. Sorry for the hassle, I should have noticed earlier when I reviewed #63279.

The final removal of the old driver can be done after v25.2 LTS.

@allmazz

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

A new MongoDB driver is now the default. Users who like to continue using the legacy driver can set server setting use_legacy_mongodb_integration to true.

[robot-clickhouse-ci-2]

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