skip day one failed case (sonic-net#6480)

What is the motivation for this PR?
skip test_syslog_source_ip, which failed consistently since day one.
How did you do it?
How did you verify/test it?
Any platform specific information?
Supported testbed topology if it's a new test case?

tests/common/plugins/conditional_mark/tests_mark_conditions.yaml

@@ -796,6 +796,12 @@ syslog/test_syslog.py:
       - "branch in ['internal-202012']"
       - "build_version <= '20201231.33'"
 
+syslog/test_syslog_source_ip.py:
+  skip:
+    reason: "Testcase consistent failed, raised issue to track"
+    conditions:
+      - https://github.com/sonic-net/sonic-mgmt/issues/6479
+
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