chore: add showedOnBoardingModal back to allowed state keys

[flotwig]

User facing changelog

n/a

Additional details

• unreleased bug uncovered in feat: updated scaffolding, new project & user banners, ability to remove scaffolding #15826
• apparently, removing a saved state key that still exists for users will cause cypress to run it through normalizeAndAllowSet again, causing the warning:
  
• followup for proper fix: saved_state utility issues warnings when keys are removed #17087

How has the user experience changed?

PR Tasks

• Have tests been added/updated?
• Has the original issue or this PR been tagged with a release in ZenHub?
• Has a PR for user-facing changes been opened in cypress-documentation?
• Have API changes been updated in the type definitions?
• Have new configuration options been added to the cypress.schema.json?

[cypress-bot]

Thanks for taking the time to open a PR!

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.
• Become familiar with the Code Review Checklist for guidelines on coding standards and what needs to be done before a PR can be merged.

[cypress]

---

Test summary

18160 • 1 • 214 • 7 • 0

---

Run details

Project	cypress
Status	Failed
Commit	97e3555
Started	Jun 23, 2021 8:17 PM
Ended	Jun 23, 2021 8:28 PM
Duration	10:47 💡
OS	Linux Debian - 10.8
Browser	Multiple

View run in Cypress Dashboard ➡️

---

Failures

	cypress/integration/commands/net_stubbing_spec.ts		1
1	network stubbing > waiting and aliasing > can timeout waiting on a single request using "alias.request"

---

This comment has been generated by cypress-bot as a result of this project's GitHub integration settings. You can manage this integration in this project's settings in the Cypress Dashboard