breaking: remove support for React 16 and 17 for Cypress Component Testing. Additionally, remove the `cypress/react18` testing harness and merge it upstream with `cypress/react`

[AtofStryker]

• Closes #29607

Additional details

Updates cypress/react to be the cypress/react18 test harness and removes support for react 16 and 17 for component testing. Since the cypress/react18 test harness is merged upstream into cypress/react, the cypress/react18 harness is no longer shipped with the binary and the cypress/react18 package will be deprecated when we release cypress 14 (a part of the release checklist)

most of the system test updates to react 18 happened in #30614 to make this PR smaller. The only changes here should be to reference the merged upstream harness, cypress/react

Steps to test

How has the user experience changed?

Test examples in the react test harness have been updated to use functional components and hooks (except 1 until react class components are completely removed)

You can also test migration via the binary https://github.com/cypress-io/cypress/commit/34043f99d989800c14f340b71f12d9d245d29a0c

• [x] what does migration look like for react 18 users? When users try to start their tests using the cypress/react18 test harness, the tests errors with cypress/react18 is not exported from cypress:

Updating cypress/react18 to cypress/react resolves the issue

• [x] what does migration look like for react 17 and under users? Users are initially warned on the launchpad that the version is not compatible

When starting Component testing with the new cypress/react harness, an error throughs that react-dom/client cannot be found

downloading @cypress/react@8 and changing the mount function from cypress/react to @cypress/react allows the tests to run

PR Tasks

• [x] Have tests been added/updated?
• [x] Has a PR for user-facing changes been opened in cypress-documentation? https://github.com/cypress-io/cypress-documentation/pull/6003
• [x] deprecate the @cypress/react18 test harness with a message that it has merged upstream (added to release checklist)
• [ ] Have API changes been updated in the type definitions?

[cypress[bot]]

cypress    Run #58448

Run Properties:   Passed #58448  •  90da84a3b5: update tests / suggestions from code review [run ci]

Project	cypress
Branch Review	breaking/remove_react_16_17_merge_react18_harness_upstream
Run status	Passed #58448
Run duration	16m 59s
Commit	90da84a3b5: update tests / suggestions from code review [run ci]
Committer	AtofStryker
View all properties for this run ↗︎

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Test results
Failures	0
Flaky	0
Pending	28
Skipped	0
Passing	764
View all changes introduced in this branch ↗︎

UI Coverage  66.27%
Untested elements	26
Tested elements	55

Accessibility  96.21%
Failed rules	0 critical   4 serious   1 moderate   0 minor
Failed elements	199