Plugin: backport PHP 8 syntax to PHP 7.4 (hc-eff)

The central history server work introduced four PHP 8.0+ syntax sites in
the plugin codebase. The main plugin file's "Requires PHP: 7.4" header
was already there; the code had silently drifted past that bound.

- includes/class-att-hc-session.php:244 — str_starts_with($host, 'www.')
  → substr($host, 0, 4) === 'www.'.
- includes/class-att-hc-api.php:52 — self::request(…, requires_auth: false)
  → positional false. Same default semantics, same callee signature.
- includes/admin-page.php:165 and class-att-hc-session.php:126 —
  list_healthchecks(…, include_steps: …, limit: …) → positional. Same
  values, no semantic change.

Verified by linting all 16 plugin files against PHP 7.4.33 — no syntax
errors, no residual PHP 8+ patterns (str_starts_with/contains/ends_with,
nullsafe, enum, readonly, mixed/never, constructor promotion, match).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

includes/admin-page.php

@@ -162,7 +162,7 @@ function att_hc_render_start_panel(): void {
     $recent     = [];
     $lookup_err = null;
     try {
-        $resp = ATT_HC_Api::list_healthchecks($site_key, include_steps: false, limit: 20);
+        $resp = ATT_HC_Api::list_healthchecks($site_key, false, 20);
         foreach ($resp['healthchecks'] ?? [] as $hc) {
             if (empty($hc['finished_at'])) $incomplete[] = $hc;
         }