Backport 'Improve Deployments and Size quota specs for clarity and consistency' to 18-7-stable-ee

What does this MR do and why?

Backport of !223900 (merged) to 18-7-stable-ee.

Improves the Gitlab::Ci::Pipeline::Quota::Deployments and Gitlab::Ci::Pipeline::Quota::Size specs for clarity and consistency:

• Uses plan_limits.destroy! instead of stubbing actual_plan to simulate "limit does not exist"
• Uses let_it_be_with_refind(:plan_limits) — necessary because destroy! freezes the AR object
• Removes deployments_spec.rb from rspec_order_todo.yml since it can now run in random order

This prevents recurring flaky test failures in CI for stable branch maintenance and patch work.

Original MR: !223900 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.