Make the deprecation a warning

Author: dkhalanskyjb

core/common/src/DateTimePeriod.kt

@@ -579,7 +579,7 @@ public fun Duration.toDateTimePeriod(): DateTimePeriod = buildDateTimePeriod(tot
  */
 @Deprecated(
     "Adding periods is not a well-defined operation. See https://github.com/Kotlin/kotlinx-datetime/issues/381",
-    level = DeprecationLevel.ERROR
+    level = DeprecationLevel.WARNING
 )
 public operator fun DateTimePeriod.plus(other: DateTimePeriod): DateTimePeriod = buildDateTimePeriod(
     safeAdd(totalMonths, other.totalMonths),
@@ -597,7 +597,7 @@ public operator fun DateTimePeriod.plus(other: DateTimePeriod): DateTimePeriod =
  */
 @Deprecated(
     "Adding periods is not a well-defined operation. See https://github.com/Kotlin/kotlinx-datetime/issues/381",
-    level = DeprecationLevel.ERROR
+    level = DeprecationLevel.WARNING
 )
 public operator fun DatePeriod.plus(other: DatePeriod): DatePeriod = DatePeriod(
     safeAdd(totalMonths, other.totalMonths),

core/common/test/DateTimePeriodTest.kt

@@ -144,7 +144,7 @@ class DateTimePeriodTest {
 
         val dp1 = DatePeriod(years = 1, months = 6)
 
-        @Suppress("DEPRECATION_ERROR")
+        @Suppress("DEPRECATION")
         run {
             assertEquals(DateTimePeriod(years = 10, days = 3, hours = 2), p1 + p2 + p3)
             assertEquals(DatePeriod(years = 11, months = 6), dp1 + p1)