Add 2026-07 as upcoming release candidate API version

Follows convention of including the next quarterly version in the supported list

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lizkenyon

lib/shopify_api/admin_versions.rb

@@ -5,6 +5,7 @@ module ShopifyAPI
   module AdminVersions
     SUPPORTED_ADMIN_VERSIONS = T.let([
       "unstable",
+      "2026-07",
       "2026-04",
       "2026-01",
       "2025-10",