Updated the integration_test page - #5693

Made the following changes:

• Changed the URLs to use the macros defined in _config.yml
• Changed the headers to use sentence case, as per our style guidelines
• Changed the link to the integration_test package to use the link in the Flutter SDK GitHub repo, rather than the pub.dev link.
• Added a note about the integration_test package moving to the Flutter SDK (and out of pub.dev) as of Flutter 2.
• Also added a note that this requires a change to the app's pubspec file.
• Finally, shortened the line lengths so that the raw file is easier for me to read.