update cloudsql proxy version to 1.21.0

Author: mattes

README.md

@@ -18,7 +18,7 @@ or use a tool like [Terraform](https://www.terraform.io).
 | `creds`                          | ***Required*** Service Account JSON Key (not base64 encoded)                |
 | `instance`                       | ***Required*** Cloud SQL connection name                                    |
 | `port`                           | Listen on port, default 5432                                                |
-| `proxy_version`                  | Cloud SQL Proxy version, default 1.18.0                                     |
+| `proxy_version`                  | Cloud SQL Proxy version, default 1.21.0                                     |
 
 
 ## Example Usage

action.yml

@@ -19,7 +19,7 @@ inputs:
   proxy_version:
     description: "CloudSQL Proxy Version"
     required: false
-    default: 1.18.0
+    default: 1.21.0
 
 runs:
   using: "composite"