[integration_test] Update README installation instructions

[jmagman]

integration_test has moved into the Flutter SDK at flutter/packages/integration_test. However, the "old instructions" iOS Device Testing instructions are wrong, which is confusing users who find the README through a Google search.

Update the README to match the instructions in https://github.com/flutter/flutter/tree/master/packages/integration_test#ios-device-testing. This is not for pub publication so won't update https://pub.dev/packages/integration_test instructions.

flutter/flutter#72876

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter]. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[stuartmorgan]

LGTM in that it's certainly better, but:

• If we're concerned about people finding outdated instructions, don't we want to publish this to pub, so there are fewer copies that are wrong?
• We're unlikely to remember to keep updating this; if these are actively confusing, how about we just remove everything but the deprecation notice (and publish that) so there's only one copy going forward?

[jmagman]

If we're concerned about people finding outdated instructions, don't we want to publish this to pub, so there are fewer copies that are wrong?

I would need to resurrect the old code to publish to pub, do we want that?

We're unlikely to remember to keep updating this; if these are actively confusing, how about we just remove everything but the deprecation notice (and publish that) so there's only one copy going forward?

That makes more sense. I can update this README to point to a link to https://github.com/flutter/flutter/tree/master/packages/integration_test.

[stuartmorgan]

I would need to resurrect the old code to publish to pub, do we want that?

Oops, I forgot I deleted it.

Let's start with just changing this copy then, and if we see evidence people are being confused by the pub.dev copy we can update the README on a branch that's split off from just before I removed the code, and publish from that.