Update gradle, kotlin and other dependencies (#2291)

* update gradle, kotlin and other dependencies

* fix new warnings

* remove unused import

* update Proguard rules

* add explicit dependency on Gson to get the newest version

* remove debug flag from proguard rules again

* fix typo

app/src/main/java/com/keylesspalace/tusky/components/timeline/viewmodel/CachedTimelineRemoteMediator.kt

@@ -33,7 +33,7 @@ import com.keylesspalace.tusky.util.dec
 import kotlinx.coroutines.rx3.await
 import retrofit2.HttpException
 
-@ExperimentalPagingApi
+@OptIn(ExperimentalPagingApi::class)
 class CachedTimelineRemoteMediator(
     accountManager: AccountManager,
     private val api: MastodonApi,

app/src/main/java/com/keylesspalace/tusky/components/timeline/viewmodel/CachedTimelineViewModel.kt

@@ -63,7 +63,7 @@ class CachedTimelineViewModel @Inject constructor(
     private val gson: Gson
 ) : TimelineViewModel(timelineCases, api, eventHub, accountManager, sharedPreferences, filterModel) {
 
-    @ExperimentalPagingApi
+    @OptIn(ExperimentalPagingApi::class)
     override val statuses = Pager(
         config = PagingConfig(pageSize = LOAD_AT_ONCE),
         remoteMediator = CachedTimelineRemoteMediator(accountManager, api, db, gson),

app/src/main/java/com/keylesspalace/tusky/components/timeline/viewmodel/NetworkTimelineRemoteMediator.kt

@@ -26,7 +26,7 @@ import com.keylesspalace.tusky.util.toViewData
 import com.keylesspalace.tusky.viewdata.StatusViewData
 import retrofit2.HttpException
 
-@ExperimentalPagingApi
+@OptIn(ExperimentalPagingApi::class)
 class NetworkTimelineRemoteMediator(
     private val accountManager: AccountManager,
     private val viewModel: NetworkTimelineViewModel

app/src/main/java/com/keylesspalace/tusky/components/timeline/viewmodel/NetworkTimelineViewModel.kt

@@ -63,7 +63,7 @@ class NetworkTimelineViewModel @Inject constructor(
 
     var nextKey: String? = null
 
-    @ExperimentalPagingApi
+    @OptIn(ExperimentalPagingApi::class)
     override val statuses = Pager(
         config = PagingConfig(pageSize = LOAD_AT_ONCE),
         pagingSourceFactory = {