refactor: migrate to kotlinx serialization, eliminate gson dependency

app/src/main/java/com/bitcointxoko/gudariwallet/api/LNbitsClient.kt

@@ -5,9 +5,10 @@ import com.bitcointxoko.gudariwallet.util.WalletConstants
 import kotlinx.coroutines.runBlocking
 import okhttp3.HttpUrl.Companion.toHttpUrl
 import okhttp3.Interceptor
+import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.OkHttpClient
 import retrofit2.Retrofit
-import retrofit2.converter.gson.GsonConverterFactory
+import retrofit2.converter.kotlinx.serialization.asConverterFactory
 import java.util.concurrent.TimeUnit
 
 object LNbitsClient {
@@ -59,8 +60,11 @@ object LNbitsClient {
         return Retrofit.Builder()
             .baseUrl("https://placeholder.invalid/")  // structurally required by Retrofit;
             .client(client)
-            .addConverterFactory(GsonConverterFactory.create(GsonProvider.gson))
-            .build()
+            .addConverterFactory(
+                JsonProvider.json.asConverterFactory(
+                    "application/json; charset=UTF-8".toMediaType()
+                )
+            )            .build()
             .create(LNbitsApi::class.java)
     }
 }