feat: include server agent API version in buildinfo (#11057)

First part of #10340 -- we need this version to compare with agents to tell if they are on a deprecated Agent API version

coderd/apidoc/docs.go

@@ -7827,6 +7827,10 @@ const docTemplate = `{
         "codersdk.BuildInfoResponse": {
             "type": "object",
             "properties": {
+                "agent_api_version": {
+                    "description": "AgentAPIVersion is the current version of the Agent API (back versions\nMAY still be supported).",
+                    "type": "string"
+                },
                 "dashboard_url": {
                     "description": "DashboardURL is the URL to hit the deployment's dashboard.\nFor external workspace proxies, this is the coderd they are connected\nto.",
                     "type": "string"

coderd/apidoc/swagger.json

@@ -6970,6 +6970,10 @@
     "codersdk.BuildInfoResponse": {
       "type": "object",
       "properties": {
+        "agent_api_version": {
+          "description": "AgentAPIVersion is the current version of the Agent API (back versions\nMAY still be supported).",
+          "type": "string"
+        },
         "dashboard_url": {
           "description": "DashboardURL is the URL to hit the deployment's dashboard.\nFor external workspace proxies, this is the coderd they are connected\nto.",
           "type": "string"

coderd/deployment.go

@@ -71,10 +71,11 @@ func (api *API) deploymentStats(rw http.ResponseWriter, r *http.Request) {
 func buildInfo(accessURL *url.URL) http.HandlerFunc {
 	return func(rw http.ResponseWriter, r *http.Request) {
 		httpapi.Write(r.Context(), rw, http.StatusOK, codersdk.BuildInfoResponse{
-			ExternalURL:    buildinfo.ExternalURL(),
-			Version:        buildinfo.Version(),
-			DashboardURL:   accessURL.String(),
-			WorkspaceProxy: false,
+			ExternalURL:     buildinfo.ExternalURL(),
+			Version:         buildinfo.Version(),
+			AgentAPIVersion: AgentAPIVersionREST,
+			DashboardURL:    accessURL.String(),
+			WorkspaceProxy:  false,
 		})
 	}
 }