v12 Migration Guide

[mjhenkes]

• Removing the [experimentalSessionAndOrigin](https://docs.cypress.io/guides/references/experiments) flag
• Test Isolation
• Need for cy.visit() in every e2e test(?)
• Moving from programmatic login to cy.origin() / cy.session()
• Detached DOM (Chaining issues?)
• Breaking changes to invoke
• Cypress.cookie.defaults & `Cypress.cookie.preserveOnce removed
• cy.route() / cy.server() and Cypress.Server.defaults replaced with cy.intercept()