PSMDB-2025 restore multiversion setup

[olexandr-havryliak]

This pull request updates the entry point of the buildscripts/setup_multiversion_mongodb.py script to execute its main functionality directly, instead of displaying a deprecation message.

Entrypoint behavior change:

• The script now calls the main() function directly when executed, removing the previous message instructing users to install and use the db-contrib-tool instead. (buildscripts/setup_multiversion_mongodb.py)

[github-actions]

Backport Policy Overview

@olexandr-havryliak, This branch (v8.0) is a production release branch and does not accept direct PRs.
All changes must flow through the corresponding staging branch (v8.0-staging), where they are tested and validated before being promoted into v8.0 by the Server Release team.

Next step: Please re-open this PR against v8.0-staging.

Please reach out in #server-release if you have any questions.