Fix the target db version of portchannel-key test (#1842)

What I did
The port-channel key migrator was introduced in version 2_0_2 so the expected database version of the test case should be 2_0_2.
It was modified to 2_0_3 when the new version was introduced by mistake. This won't fail the test but disable the require its database version to be updated every time a new version is introduced. (Refer #1566 and #1614 for details)
This is to correct it by changing it back to 2_0_2.

Signed-off-by: Stephen Sun <stephens@nvidia.com>

tests/db_migrator_input/config_db/portchannel-expected.json

@@ -33,7 +33,7 @@
         "lacp_key": "auto"
     },
     "VERSIONS|DATABASE": {
-        "VERSION": "version_2_0_3"
+        "VERSION": "version_2_0_2"
     }
 }