On Unity 2018 or lower, how do I resolve the Legacy Build System error on Xcode?

When exporting the project to Xcode, if you see the following build error:

"The Legacy Build System will be removed in a future release. You can configure the selected build system and this deprecation message in File → Project Settings"

Follow these steps to resolve this issue:

1. Select File → Project Settings.
   
2. Under Shared Project Settings, change the Build System settings to 'New Build System'.
   
3. Click Done and rebuild your project.