RUMM-1758 Fix Example app compilation in Xcode 12

[ncreated]

What and why?

🐞⚙️ This PR fixes Example compilation problem in Xcode 12 (used in nightly tests).

The issue was introduced in SwiftUI code added in #669, where we use a construct not available for Xcode 12.

How?

Just replacing:

- .listStyle(.plain)
+ .listStyle(PlainListStyle())

and not shadowing class fields with local variables:

- let viewKey = viewKey
+ let key = viewKey

I also migrated our xcworkspace by dismissing recommended changes for Xcode 13.1 (none of them was applicable in our setup).

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference

[buranmert]

curious: is the rest of the PR necessary for the fix?

[ncreated]

Yes, in some CI logs from Xcode 12 I can also see:

❌  /Users/vagrant/git/Datadog/Example/Debugging/DebugRUMSessionViewController.swift:47:23: variable used within its own initial value
        let viewKey = viewKey
                      ^

Some other failed with:

▸ Running script '⚙️ Run linter'
Automatic retry reason found in log: Early unexpected exit, operation never finished bootstrapping - no restart will be attempted
isAutomaticRetryOnReason=false, no more retry, stopping the test!

and I don't yet have a clue on what's wrong, but it seems unrelated to #669 changes.

Although dismissing Xcode's "upgrade workspace to recommended changes" warning won't fix it I played a boy-scout to cleanup our logs.

[maxep]

LGTM