Update Proguard rules for R8 fullmode

Taken from here https://github.com/JakeWharton/SdkSearch/commit/498ab309cf83e47e2546379dc1c59eac27711bec

retrofit/src/main/resources/META-INF/proguard/retrofit2.pro

-# Retrofit does reflection on generic parameters and InnerClass is required to use Signature.
--keepattributes Signature, InnerClasses
+# Retrofit does reflection on generic parameters. InnerClasses is required to use Signature and
+# EnclosingMethod is required to use InnerClasses.
+-keepattributes Signature, InnerClasses, EnclosingMethod
 
 # Retain service method parameters when optimizing.
 -keepclassmembers,allowshrinking,allowobfuscation interface * {