Point kotlin message in `gradle_errors.dart` towards new place where templates define the kotlin version

[gmackall]

Fixes https://github.com/flutter/flutter/issues/144454

For reference, after formatting it looks like:

┌─ Flutter Fix ──────────────────────────────────────────────────────────────────────────────────────────┐
│ [!] Your project requires a newer version of the Kotlin Gradle plugin.                                 │
│ Find the latest version on https://kotlinlang.org/docs/releases.html#release-details, then update the  │
│ version number of the plugin with id "org.jetbrains.kotlin.android" in the plugins block of            │
│ /Users/mackall/development/BugTesting/ELIMINATEME/blah/android/settings.gradle.                        │
│                                                                                                        │
│ Alternatively (if your project was created before Flutter 3.19), update                                │
│ /Users/mackall/development/BugTesting/ELIMINATEME/blah/android/build.gradle                            │
│ ext.kotlin_version = '<latest-version>'                                                                │
└────────────────────────────────────────────────────────────────────────────────────────────────────────┘

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• [x] I signed the CLA.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] I followed the breaking change policy and added Data Driven Fixes where supported.
• [x] All existing and new tests are passing.

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