CHANGELOG: add advisory on multiproject support

On HCP, multiproject support was GA'd recently, but Packer does not
support the feature yet.

To avoid clients being surprised if they encounter an error with this
feature enabled, we add an advisory related to this feature not being
supported for now, but planned for later.

CHANGELOG.md

@@ -67,6 +67,15 @@ if using HCL2, a `required_plugins` block to define a list of plugins to use for
 * Bumped gopsutil to v3. This fixes a macOS intermittent crash reported by the community
     [GH-12430](https://github.com/hashicorp/packer/issues/12430)
 
+### HCP Packer -- Multi project support advisory
+
+If using HCP Packer to store metadata on your builds, please be aware that Packer
+does not support this feature yet, and as such, having multiple projects in your
+organization will make it impossible to use HCP Packer in this release.
+
+We are actively working on supporting this feature, which will be available in a
+subsequent release.
+
 ## 1.8.7 (May 4, 2023)
 
 ### NOTES: