chore: bump wordpress to 7.1

[YvesCesar]

The published image ships WordPress 6.9.4, but woocommerce-nextcloud-admin-group-manager requires WordPress 7.0. On LibreSign SaaS, the plugin cannot be activated:

Current WordPress version (6.9.4) does not meet minimum requirements for WooCommerce Nextcloud Admin Group Manager. The plugin requires WordPress 7.0.

Changes

• VERSION_WORDPRESS in the publish workflow goes from 6.9 to 7.1, the current WordPress release.
• The commented build default in common-services.yml follows it.

Validation

• Built the image locally with VERSION_WORDPRESS=7.1 and VERSION_PHP=8.3. The wordpress:7.1-php8.3-fpm base image exists.
• On a fresh LibreSign SaaS clone, the entrypoint installed all configured plugins and themes. woocommerce-nextcloud-admin-group-manager activated, and the SaaS integration tests passed.

Notes

• Existing installs keep the WordPress core in their volume. Only fresh installs get 7.1 automatically; existing ones need wp core update.
• Related: fix: start the stack from a fresh clone LibreSign/saas#24.