Add missing @SInCE, setting it straight to 28.1.

RELNOTES=n/a

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=265963602

Author: cgdecker

guava/src/com/google/common/collect/RangeMap.java

@@ -121,6 +121,8 @@ public interface RangeMap<K extends Comparable, V> {
    * <p>For example, if {@code rangeMap} had one entry {@code [1, 5] => 3} then {@code
    * rangeMap.merge(Range.closed(0,2), 3, Math::max)} could yield a range map with the entries
    * {@code [0, 1) => 3, [1, 2] => 3, (2, 5] => 3}.
+   *
+   * @since 28.1
    */
   void merge(
       Range<K> range,