[utils] mark update-verify-tests tests unsupported with old Python

This utility uses the walrus operator introduced in Python 3.8, but this
causes tests to fail in Amazon Linux 2 CI, which uses Python 3.7. Since
this is not a shipped product, that doesn't really matter, and 3.7 is an
ancient EOL version anyways. The premerge CI uses Python 3.9 and later,
so use that as a basis, just to avoid the same issue in the future in
case some Python 3.9 feature sneaks in.

rdar://164151931

Author: hnrklssn

test/Utils/update-verify-tests/lit.local.cfg

@@ -0,0 +1,4 @@
+import sys
+
+if sys.version_info < (3, 9):
+    config.unsupported = True