Bump 3.6-rspm snapshot to after IRkernel was introduced

yuvipanda · yuvipanda · commit 12240843457d · 2023-06-09T17:56:42.000+05:30
diff --git a/tests/r/r3.6-rspm/runtime.txt b/tests/r/r3.6-rspm/runtime.txt
@@ -1 +1 @@
-r-3.6-2017-11-01
+r-3.6-2018-11-20
