test: engine version enforcement

ref: 5842

Signed-off-by: Chin-Ya Huang <chin-ya.huang@suse.com>

docs/content/manual/release-specific/v1.6.0/test-engine-version-enforcement.md

@@ -0,0 +1,42 @@
+---
+title: Test engine version enforcement
+---
+
+## Related issue
+https://github.com/longhorn/longhorn/issues/5842
+
+## Test step
+
+**Given** Longhorn v1.4.x cluster running  
+And create and attach a volume (volume-1)  
+And upgraded Longhorn to v1.5.x  
+And create and attach a volume (volume-2)
+
+**When** upgraded Longhorn to v1.6.0  
+**Then** v1.6.0 longhorn-manager Pods should be in crashloop
+```
+longhorn-manager-zrf8r                                0/1     CrashLoopBackOff   2 (10s ago)   52s
+longhorn-manager-zsph2                                0/1     CrashLoopBackOff   2 (8s ago)    52s
+longhorn-manager-grhsf                                0/1     CrashLoopBackOff   2 (8s ago)    51s
+```
+And should see incompatible version error in longhorn-manager Pod logs
+```
+time="2023-08-17T03:03:20Z" level=fatal msg="Error starting manager: failed checking Engine upgarde path: incompatible Engine ei-7fa7c208 client API version: found version 7 is below required minimal version 8"
+```
+
+**When** downgraded Longhorn to v1.5.x  
+**Then** Longhorn components should be running
+
+**When** upgraded v1.4.1 volume (volume-1) engine  
+And upgraded Longhorn to v1.6.0
+**Then** Longhorn components should be running  
+And v1.4.x EngineImage state should be incompatible
+```
+NAME          STATE          IMAGE                               REFCOUNT   BUILDDATE   AGE
+ei-74783864   deployed       longhornio/longhorn-engine:v1.5.1   10         28d         12m
+ei-7fa7c208   incompatible   longhornio/longhorn-engine:v1.4.1   0          157d        13m
+ei-ad420081   deployed       c3y1huang/research:2017-lh-ei       0          44h         24s
+```
+
+**When** update existing volume/engine/replica custom resourcs `spec.image` with `longhornio/longhorn-engine:v1.4.x`  
+**Then** should be blocked