`productSettingsXML` unrecognized

```
[!] Xcodeproj doesn't know about the following attributes {"productSettingsXML"=>"\n\n\n\n\tCFBundleDevelopmentRegion\n\t$(DEVELOPMENT_LANGUAGE)\n\tCFBundleExecutable\n\t$(EXECUTABLE_NAME)\n\tCFBundleIdentifier\n\t$(PRODUCT_BUNDLE_IDENTIFIER)\n\tCFBundleInfoDictionaryVersion\n\t6.0\n\tCFBundleName\n\t$(PRODUCT_NAME)\n\tCFBundlePackageType\n\t$(PRODUCT_BUNDLE_PACKAGE_TYPE)\n\tCFBundleShortVersionString\n\t$(MARKETING_VERSION)\n\tCFBundleVersion\n\t$(CURRENT_PROJECT_VERSION)\n\n\n"} for the 'PBXLegacyTarget' isa.
If this attribute was generated by Xcode please file an issue: https://github.com/CocoaPods/Xcodeproj/issues/new
```

Note: I am experiencing this as a result of running `fastlane` against my project.

As a consequence of this, the `productSettingsXML` value in my `xcodeproj` is deleted.

Xcode brings it back whenever I modify the build settings.

The consequence is a lot of change noise to my `xcodeproj` file, going back-and-forth between adding and deleting this section. This also messes with my release process.

Start by tracing how PBXLegacyTarget attributes are parsed and serialized in Xcodeproj, then inspect any tests covering unknown or generated project attributes. Reproduce the fastlane-related round trip and verify that productSettingsXML is preserved without repeated changes to the xcodeproj file.

Written by the indexing model from the issue text.