Disable flaky test in ExoPlayerTest

targetLiveOffsetInMedia_unknownWindowStartTime_doesNotAdjustLiveOffset
is flaky on the release branch. It's not flaky on the dev-v2 branch
due to recent changes to FakeClock's behaviour which aren't yet present
on release-v2.

library/core/src/test/java/com/google/android/exoplayer2/ExoPlayerTest.java

@@ -8836,6 +8836,7 @@ public final class ExoPlayerTest {
   }
 
   @Test
+  @Ignore("This is flaky in the release branch. It's stable on dev-v2 due to FakeClock changes.")
   public void targetLiveOffsetInMedia_unknownWindowStartTime_doesNotAdjustLiveOffset()
       throws Exception {
     FakeClock fakeClock = new AutoAdvancingFakeClock(/* initialTimeMs= */ 987_654_321L);