Change release archive check to warning - #57688

Description

I believe we've tracked down most of the issues related to accidentally archiving with a Debug version of Flutter. Switch the error to a warning to accommodate users who want to distribute Debug versions to QA, etc.

echoing the message with a warning: prefix makes it a real Xcode warning.

Before:

After:

Related Issues

Fixes #57557.

Tests

We don't archive anywhere, not sure how to test this...

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.