chore: remove coder_secret Terraform integration (#25512)

Removes the coder_secret Terraform integration: the data.coder_secret
consumption path through provisionerdserver → provisioner.proto →
provisioner/terraform, the dynamic-parameter secret-requirement
validation, and the workspace-update / resolve-autostart surfaces that
depended on it. This is being done due to a product/feature direction
change (see PLAT-243). User-secret CRUD (DB, REST, CLI, UI, telemetry, audit)
and the agent-manifest secret-injection path are untouched.

The provisionerd API is bumped from v1.17 to v1.18 rather than rolled
back: v1.17 shipped in v2.33.x, so user_secrets field numbers are
reserved and the changelog documents both versions.

Generated with assistance from Coder Agents.

docs/reference/api/templates.md

@@ -2802,14 +2802,6 @@ curl -X POST http://coder-server:8080/api/v2/templateversions/{templateversion}/
         "value": "string"
       }
     }
-  ],
-  "secret_requirements": [
-    {
-      "env": "string",
-      "file": "string",
-      "help_message": "string",
-      "satisfied": true
-    }
   ]
 }
 ```