Atlassian is planning to enforce asymmetric JWTs to all apps
install/uninstall lifecycle events by Oct 29, 2021

To verify the asymmetric JWT we must fetch a public_key from
a Atlassian CDN.

Read more about this change here:
https://community.developer.atlassian.com/t/action-required-atlassian-connect-installation-lifecycle-security-improvements/49046

Changelog: changed