Replace identical InstantSerializer with KotlinX datetime default serializer

Author: SimonMarquis

core/network/src/main/java/com/google/samples/apps/nowinandroid/core/network/model/NetworkNewsResource.kt

@@ -17,7 +17,6 @@
 package com.google.samples.apps.nowinandroid.core.network.model
 
 import com.google.samples.apps.nowinandroid.core.model.data.NewsResource
-import com.google.samples.apps.nowinandroid.core.network.model.util.InstantSerializer
 import kotlinx.datetime.Instant
 import kotlinx.serialization.Serializable
 
@@ -31,7 +30,6 @@ data class NetworkNewsResource(
     val content: String,
     val url: String,
     val headerImageUrl: String,
-    @Serializable(InstantSerializer::class)
     val publishDate: Instant,
     val type: String,
     val topics: List<String> = listOf(),
@@ -47,7 +45,6 @@ data class NetworkNewsResourceExpanded(
     val content: String,
     val url: String,
     val headerImageUrl: String,
-    @Serializable(InstantSerializer::class)
     val publishDate: Instant,
     val type: String,
     val topics: List<NetworkTopic> = listOf(),