Upgrade kotlin to 1.6.21

Also fix a collection of warnings.

app/src/main/java/org/thoughtcrime/securesms/util/rx/ReactiveTask.kt

@@ -1,19 +0,0 @@
-package org.thoughtcrime.securesms.util.rx
-
-import com.google.android.gms.tasks.Task
-import io.reactivex.rxjava3.core.Single
-
-/**
- * Convert a [Task] into a [Single].
- */
-fun <T : Any> Task<T>.toSingle(): Single<T> {
-  return Single.create { emitter ->
-    addOnCompleteListener {
-      if (it.isSuccessful && !emitter.isDisposed) {
-        emitter.onSuccess(it.result)
-      } else if (!emitter.isDisposed) {
-        emitter.onError(it.exception)
-      }
-    }
-  }
-}