Nudge issue filers to repo in vanilla SwiftUI (#1562)

* Nudge issue filers to repo in vanilla SwiftUI

Gently remind filers to attempt to repro issues in vanilla SwiftUI, rather than imply that such issues shouldn't be filed.

* Make reproducing in vanilla SwiftUI a non-required checklist item

Author: gohanlon

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -21,8 +21,8 @@ body:
   attributes:
     label: Checklist
     options:
-    - label: I have not been able to reproduce this bug in a vanilla SwiftUI project.
-      required: true
+    - label: I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
+      required: false
     - label: If possible, I've reproduced the issue using the `main` branch of this package.
       required: false
     - label: This issue hasn't been addressed in an [existing GitHub issue](https://github.com/pointfreeco/swift-composable-architecture/issues) or [discussion](https://github.com/pointfreeco/swift-composable-architecture/discussions).