[RUN AS-IF-FOSS] Replace calls to `Timecop.freeze` with `travel_to`

What does this MR do?

In !39656 (merged), we added a cop to detect usages of Timecop.freeze. Before this cop can be enabled, we need to fix existing offences in the codebase.

This MR replaces some straightforward calls to Timecop.freeze in the spec directory. Since freeze_time (the recommended replacement for Timecop.freeze) does not take any arguments, the replacement for Timecop.freeze with a specific time is travel_to.

There are still some calls to Timecop.freeze leftover which are tricky to replace. In a future MR, these will be added to .rubocop_todo.yml and dealt with on a case-by-case basis.

Related to #214432 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team