chore(release): bump version to 1.6.0

The Phase 1 + Phase 2 KEK/DEK envelope-encryption rewrite landed across
commits 7d96aee..36b8829 but never bumped Directory.Build.props. Stamping
the right version on the binaries so WorkerHeartbeat.Version, Graph
User-Agent, and audit entries report 1.6.0 instead of the stale 1.5.4.

Build 0/0, tests 50/50.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: robgrame

Directory.Build.props

@@ -6,10 +6,10 @@
     row's Version column, and the published assembly stamps in one place.
   -->
   <PropertyGroup>
-    <Version>1.5.4</Version>
-    <AssemblyVersion>1.5.4.0</AssemblyVersion>
-    <FileVersion>1.5.4.0</FileVersion>
-    <InformationalVersion>1.5.4</InformationalVersion>
+    <Version>1.6.0</Version>
+    <AssemblyVersion>1.6.0.0</AssemblyVersion>
+    <FileVersion>1.6.0.0</FileVersion>
+    <InformationalVersion>1.6.0</InformationalVersion>
 
     <Company>robgrame</Company>
     <Product>BitLocker Key Monitor</Product>