Fix vulnerability related to old node fetch

[yizheliu-amazon]

https://github.com/opendistro-for-elasticsearch/anomaly-detection-kibana-plugin/network/alert/yarn.lock/node-fetch/open

 Dependabot cannot update node-fetch to a non-vulnerable version
The latest possible version that can be installed is 1.7.3 because of the following conflicting dependency:

formik@1.5.8 requires node-fetch@^1.0.1 via a transitive dependency on isomorphic-fetch@2.2.1
The earliest fixed version is 2.6.1.