Add missing License details for Connexion (apache#15906)

This was missed in apache#15781

Since "connexion" is Apache licensed, this is "not a blocker" for 2.1.0 as mentioned in https://www.apache.org/legal/release-policy.html#license-file

>When a package bundles code under several licenses, the LICENSE file MUST contain details of all these licenses. For each component which is not Apache licensed, details of the component MUST be appended to the LICENSE file. The component license itself MUST either be appended or else stored elsewhere in the package with a pointer to it from the LICENSE file, e.g. if the license is long.

As "connextion" is Apache 2 Licensed, this _might_ be OK.

LICENSE

@@ -220,6 +220,7 @@ at licenses/LICENSE-[project].txt.
     (ALv2 License) hue v4.3.0 (https://github.com/cloudera/hue/)
     (ALv2 License) jqclock v2.3.0 (https://github.com/JohnRDOrazio/jQuery-Clock-Plugin)
     (ALv2 License) bootstrap3-typeahead v4.0.2 (https://github.com/bassjobsen/Bootstrap-3-Typeahead)
+    (ALv2 License) connexion v2.7.0 (https://github.com/zalando/connexion)
 
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 MIT licenses