Add `@DeprecatedSinceMirai`, close #1669

mirai-core-utils/src/commonMain/kotlin/Annotations.kt

@@ -15,4 +15,13 @@ import kotlin.annotation.AnnotationTarget.*
 
 @RequiresOptIn("This can only be used in tests.", level = ERROR)
 @Target(CLASS, FUNCTION, PROPERTY, CLASS, CONSTRUCTOR, FUNCTION)
-public annotation class TestOnly
+public annotation class TestOnly
+
+/**
+ * @since 2.9.0-RC
+ */ // https://github.com/mamoe/mirai/issues/1669
+public annotation class DeprecatedSinceMirai(
+    val warningSince: String = "",
+    val errorSince: String = "",
+    val hiddenSince: String = ""
+)