Fix: widening return types is not breaking

[devmil]

Description

Widening the return type was not treated as breaking but consider this scenario:

// old signature
String someMethod();

// new signature
Object someMethod();

//-- breaks those consumer code situations:
// 1
String someVar = someMethod();
// 2
final someVar = someMethod();
final len = someVar.length;

So every change in return type needs to be breaking!

Fixes #186

Type of Change

• 🚀 New feature (non-breaking change)
• 🛠️ Bug fix (non-breaking change)
• ⚠️ Breaking change (feature or bug fix which breaks existing behaviors/APIs)
• 🏗️ Code refactor
• ⚙️ Build configuration change
• 📝 Documentation
• 🧹 Chore / Housekeeping

[devmil]

the current version already should treat widening return types as breaking and narrowing should not be breaking (at least we were not able to come up with use cases that narrow the return type and are breaking)