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

coderd/apidoc/swagger.json

@@ -7418,6 +7418,10 @@
           "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+          "description": "UpgradeMessage is the message displayed to users when an outdated client\nis detected.",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"
@@ -8079,6 +8083,9 @@
         "cache_directory": {
           "type": "string"
         },
+        "cli_upgrade_message": {
+          "type": "string"
+        },
         "config": {
           "type": "string"
         },