test(audience-sample): scope log-pane suppression to Unity 6 Linux only

Probe PR #767 showed Unity 2021.3 Linux cells regressed by 38s-2:24 with
the suppression in place; Unity 6 Linux cells improved by ~7 min. Add
UNITY_6000_0_OR_NEWER to the #if so 2021.3 and 2022.3 Linux cells are
unaffected.

Author: ImmutableJeffrey

examples/audience/Assets/SampleApp/Tests/Runtime/LinuxLogPaneSuppression.cs

@@ -1,6 +1,12 @@
 #nullable enable
 
-#if UNITY_STANDALONE_LINUX
+// Gated on Unity 6 specifically. Probe PR #767 (revert this fixture)
+// showed Unity 2021.3 Linux cells got faster without the suppression
+// (Mono -38 s, IL2CPP -2:24): the per-test SceneLoaded handler cost
+// exceeds the rendering gain on cells that are already fast. Unity 6
+// Linux cells got slower without it (Mono +7:06, IL2CPP +5:55): the
+// suppression is a real ~7 min win there. So scope it to Unity 6 only.
+#if UNITY_STANDALONE_LINUX && UNITY_6000_0_OR_NEWER
 using NUnit.Framework;
 using UnityEngine;
 using UnityEngine.SceneManagement;