Replace opendistro-alerting-1.13.1.0.zip with an external download

[dblock]

Coming from opensearch-project/opensearch-build#1750, replace the checked in alerting/alerting/src/test/resources/bwc/alerting/1.13.1.0/opendistro-alerting-1.13.1.0.zip with an external download during bwc test if it's still used.

[lezzago]

This would also need change the notification zips to be retrieved for an external download. This is going to be difficult currently since these zips will need to be the latest versions of the notification zips as the version gets bumped over time.

[dblock]

@lezzago Is this more complicated than opensearch-project/cross-cluster-replication#366 or opensearch-project/sql#514? We now have latest supported in URLs.

[qreshi]

PR has been merged and backported to 2.0 branch as well. Closing.