Use the same Rack version as the monolith

What does this MR do and why?

Use the same Rack version as the monolith

If we don't limit the Rack version in gitlab-safe_request_store, it'll be resolved to Rack 3 (latest). However, in the monolith where we're actually using the gem, we're forced to use Rack 2 because of compatibility issue: #396273

This means we're not really testing with the Rack version we're really using in the monolith.

This should make this consistent and always test against Rack 2.

See also https://gitlab.com/gitlab-com/gl-security/security-assurance/governance-and-field-security/field-security-team/customer-assurance-activities/caa-servicedesk/-/issues/3185#note_1704249947

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