m: remove references to LUK, GUK

This unifies the unlock key nomenclature throughout the repository:
- Local Unlock Key becomes a Node Unlock Key
- Global Unlock Key becomes a Cluster Unlock Key

Change-Id: I674ad68a50b3845705f3e2c57952fc7fba5be665
Reviewed-on: https://review.monogon.dev/c/monogon/+/816
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/handbook/src/ch03-00-cluster-architecture.md b/metropolis/handbook/src/ch03-00-cluster-architecture.md
index 9f5669c..68c9384 100644
--- a/metropolis/handbook/src/ch03-00-cluster-architecture.md
+++ b/metropolis/handbook/src/ch03-00-cluster-architecture.md
@@ -54,7 +54,7 @@
 '-------------------'           | Seals/Unseals
          | Mounts               v
          |           .---------------------.        .------------------------.
-         | .---------| Node Encryption Key |        |    Running Cluster     |
+         | .---------|   Node Unlock Key   |        |    Running Cluster     |
          |/          '---------------------'        |------------------------|
          | .----------------------------------------| Cluster Encryption Key |
          |/                                         |       (per node)       |