Remove @OptIn annotations that are now unnecessary

app/core/src/main/java/com/fsck/k9/notification/NotificationData.kt

@@ -17,7 +17,6 @@ internal data class NotificationData(
     val isSingleMessageNotification: Boolean
         get() = activeNotifications.size == 1
 
-    @OptIn(ExperimentalStdlibApi::class)
     val messageReferences: List<MessageReference>
         get() {
             return buildList(capacity = newMessagesCount) {

app/core/src/main/java/com/fsck/k9/notification/SingleMessageNotificationDataCreator.kt

@@ -41,7 +41,6 @@ internal class SingleMessageNotificationDataCreator {
         )
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
     private fun createSingleNotificationActions(): List<NotificationAction> {
         return buildList {
             add(NotificationAction.Reply)
@@ -53,7 +52,6 @@ internal class SingleMessageNotificationDataCreator {
         }
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
     private fun createSingleNotificationWearActions(account: Account): List<WearNotificationAction> {
         return buildList {
             add(WearNotificationAction.Reply)

app/core/src/main/java/com/fsck/k9/notification/SummaryNotificationDataCreator.kt

@@ -43,7 +43,6 @@ internal class SummaryNotificationDataCreator(
         )
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
     private fun createSummaryNotificationActions(): List<SummaryNotificationAction> {
         return buildList {
             add(SummaryNotificationAction.MarkAsRead)
@@ -54,7 +53,6 @@ internal class SummaryNotificationDataCreator(
         }
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
     private fun createSummaryWearNotificationActions(account: Account): List<SummaryWearNotificationAction> {
         return buildList {
             add(SummaryWearNotificationAction.MarkAsRead)

app/core/src/test/java/com/fsck/k9/notification/SummaryNotificationDataCreatorTest.kt

@@ -266,7 +266,6 @@ class SummaryNotificationDataCreatorTest {
         return NotificationData(account, activeNotifications, inactiveNotifications = emptyList())
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
     private fun createNotificationDataWithMultipleMessages(times: Int = 2): NotificationData {
         val contentList = buildList {
             repeat(times) {