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.

coderd/testdata/parameters/secret_required/main.tf

@@ -1,12 +0,0 @@
-terraform {
-  required_providers {
-    coder = {
-      source = "coder/coder"
-    }
-  }
-}
-
-data "coder_secret" "gh" {
-  env          = "GITHUB_TOKEN"
-  help_message = "Add a GitHub PAT with env=GITHUB_TOKEN"
-}