docs: update README integration test instructions

[paulwellnerbou]

Description

Updates the README "Integration Tests" section, which was outdated:

• Removed reference to the no-longer-existing npm run e2e script.
• Removed the old Nightwatch/Selenium (JDK 7+) instructions, since the project migrated to Cypress.
• Documented the current Cypress-based commands:
  • npm run cy:ci — full headless suite (starts servers, runs Cypress, tears them down); matches CI.
  • npm run cy:dev — interactive Cypress runner.
  • Running a single spec headless via npm run cy:start + npm run cy:run -- --spec "<path>".

Motivation and Context

New contributors following the README couldn't run the e2e suite locally because the documented command no longer exists. Fixes #10915.

How Has This Been Tested?

Documentation-only change. Verified the documented commands locally (cy:ci, and the single-spec invocation through cy:run -- --spec).

Screenshots (if appropriate):

N/A

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• ❌ Dependency changes (any modification to dependencies in package.json)
• ❌ Bug fixes (non-breaking change which fixes an issue)
• ❌ Improvements (misc. changes to existing features)
• ❌ Features (non-breaking change which adds functionality)

My changes...

• ❌ are breaking changes to a public API (config options, System API, major UI change, etc).
• ❌ are breaking changes to a private API (Redux, component props, utility functions, etc.).
• ❌ are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• ❌ My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• ❌ My changes can and should be tested by unit and/or integration tests.
• ❌ If yes to above: I have added tests to cover my changes.
• ❌ If yes to above: I have taken care to cover edge cases in my tests.
• ❌ All new and existing tests passed.