Pin multi-version upgrade schema to 16.7.6 in 16-11-stable-ee

What does this MR do and why?

The db:migrate:multi-version-upgrade CI jobs started failing in 16-11-stable-ee because the floating dependency on the schemas generated by https://gitlab.com/gitlab-org/quality/pg-dump-generator updated to a GitLab 17.x schema, which no longer has the users.u2f_registrations foreign key.

We can avoid this failure by pinning the version for this stable branch.

Relates to gitlab-org/release/tasks#13031 (closed)

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