chore: add changelog for v8.4.0 agent release

Author: hwartig

platform/private-locations/change-log.mdx

@@ -16,6 +16,12 @@ Learn more about:
 
 ## Full release history
 
+<Update label="v8.4.0" tags={["July 2026"]}>
+- Support for running gRPC, traceroute, and SSL checks
+- Always scrub potentially sensitive headers (Authorization, x-api-key, cookie, etc) even if they don’t contain secrets
+- Downgrade one of the dependencies that prevented the agent from running on older CPU hardware
+</Update>
+
 <Update label="v8.3.4" tags={["June 2026"]}>
 - Update various dependencies to fix vulnerabilities
 - Add `--refresh-cache` functionality