update link in istioHardened.md

General MR

Summary

Link was broken because the linked document was actually deleted. Linking the document in an old commit for now, and we'll remove the doc altogether after we've migrated this package to bb-common.

Relevant logs/screenshots

(Include any relevant logs/screenshots)

Linked Issue

Closes #232 (closed).

Upgrade Notices

(Include any relevant notes about upgrades here or write "N/A" if there are none)