Backport of Add upgrade warnings into release/1.16.x (#19062)

* backport of commit ebdb3f1

* backport of commit 86ff649

---------

Co-authored-by: Chris S. Kim <[email protected]>

website/content/docs/upgrading/upgrade-specific.mdx

@@ -16,11 +16,20 @@ upgrade flow.
 
 ## Consul 1.16.x
 
-#### Known issues
+### Known issues
 
 Service mesh in Consul versions 1.16.0 and 1.16.1 may have issues when a snapshot restore is performed and the servers are hosting xDS streams.
 When this bug triggers, it causes Envoy to incorrectly populate upstream endpoints. To prevent this issue, service mesh users who run agent-less workloads should upgrade Consul to v1.16.2 or later.
 
+#### Vault Enterprise as CA ((#vault-enterprise-as-ca-1-16))
+Using Vault as CA with Consul version 1.16.2 will fail to initialize the CA if [`namespace`](/consul/docs/connect/ca/vault#namespace) is set
+but [`intermediate_pki_namespace`](/consul/docs/connect/ca/vault#intermediatepkinamespace) or [`root_pki_namespace`](/consul/docs/connect/ca/vault#rootpkinamespace)
+are empty. This is a bug which will be fixed in a future version.
+
+To work around this issue, users must explicitly set [`intermediate_pki_namespace`](/consul/docs/connect/ca/vault#intermediatepkinamespace) and
+[`root_pki_namespace`](/consul/docs/connect/ca/vault#rootpkinamespace) to the same value as [`namespace`](/consul/docs/connect/ca/vault#namespace).
+Set your configuration by calling [set-config](/consul/commands/connect/ca#set-config) then use [get-config](/consul/commands/connect/ca#get-config) to check.
+
 #### API health endpoints return different status code
 
 Consul versions 1.16.0+ now return an error 403 "Permission denied" status
@@ -39,9 +48,10 @@ This change removes the backward-compatibility behavior introduced in Consul 1.1
 
 ## Consul 1.15.x
 
-#### Service mesh compatibility ((#service-mesh-compatibility-1-15))
+### Service mesh compatibility ((#service-mesh-compatibility-1-15))
 
 Upgrade to **Consul version 1.15.2 or later**.
+If using [Vault Enterprise as CA](#vault-enterprise-as-ca-1-15), **avoid Consul version 1.15.6**.
 
 Consul versions 1.15.0 - 1.15.1 contain a race condition that can cause
 some service instances to lose their ability to communicate in the mesh after
@@ -50,6 +60,15 @@ due to a problem with leaf certificate rotation.
 
 This bug is fixed in Consul versions 1.15.2 and newer.
 
+#### Vault Enterprise as CA ((#vault-enterprise-as-ca-1-15))
+Using Vault as CA with Consul version 1.15.6 will fail to initialize the CA if [`namespace`](/consul/docs/connect/ca/vault#namespace) is set
+but [`intermediate_pki_namespace`](/consul/docs/connect/ca/vault#intermediatepkinamespace) or [`root_pki_namespace`](/consul/docs/connect/ca/vault#rootpkinamespace)
+are empty. This is a bug which will be fixed in a future version.
+
+To work around this issue, users must explicitly set [`intermediate_pki_namespace`](/consul/docs/connect/ca/vault#intermediatepkinamespace) and
+[`root_pki_namespace`](/consul/docs/connect/ca/vault#rootpkinamespace) to the same value as [`namespace`](/consul/docs/connect/ca/vault#namespace).
+Set your configuration by calling [set-config](/consul/commands/connect/ca#set-config) then use [get-config](/consul/commands/connect/ca#get-config) to check.
+
 #### Removing configuration options
 
 The `connect.enable_serverless_plugin` configuration option was removed. Lambda integration is now enabled by default.
@@ -191,6 +210,14 @@ to use TLS for contacting the HTTP API, it will also incorrectly enable TLS for
 Users should not upgrade to 1.14.0 if they are using plaintext gRPC connections in
 conjunction with TLS-encrypted HTTP APIs.
 
+#### Vault Enterprise as CA ((#vault-enterprise-as-ca-1-14))
+Using Vault as CA with Consul version 1.14.10 will fail to initialize the CA if [`namespace`](/consul/docs/connect/ca/vault#namespace) is set
+but [`intermediate_pki_namespace`](/consul/docs/connect/ca/vault#intermediatepkinamespace) or [`root_pki_namespace`](/consul/docs/connect/ca/vault#rootpkinamespace)
+are empty. This is a bug which will be fixed in a future version.
+
+To work around this issue, users must explicitly set [`intermediate_pki_namespace`](/consul/docs/connect/ca/vault#intermediatepkinamespace) and
+[`root_pki_namespace`](/consul/docs/connect/ca/vault#rootpkinamespace) to the same value as [`namespace`](/consul/docs/connect/ca/vault#namespace).
+Set your configuration by calling [set-config](/consul/commands/connect/ca#set-config) then use [get-config](/consul/commands/connect/ca#get-config) to check.
 
 #### Changes to gRPC TLS configuration