feat: add customizable upgrade message on client/server version mismatch (#11587)

coderd/apidoc/docs.go

@@ -8329,6 +8329,10 @@ const docTemplate = `{
                     "description": "ExternalURL references the current Coder version.\nFor production builds, this will link directly to a release. For development builds, this will link to a commit.",
                     "type": "string"
                 },
+                "upgrade_message": {
+                    "description": "UpgradeMessage is the message displayed to users when an outdated client\nis detected.",
+                    "type": "string"
+                },
                 "version": {
                     "description": "Version returns the semantic version of the build.",
                     "type": "string"
@@ -9043,6 +9047,9 @@ const docTemplate = `{
                 "cache_directory": {
                     "type": "string"
                 },
+                "cli_upgrade_message": {
+                    "type": "string"
+                },
                 "config": {
                     "type": "string"
                 },