Set the HPOS-related features back to "experimental" (#39520)

We determined that there are a few more issues that need to be resolved before HPOS can be classified as a "mature" feature.

plugins/woocommerce/changelog/update-hpos-feature-experimental

@@ -0,0 +1,5 @@
+Significance: patch
+Type: update
+Comment: Reverting HPOS to an experimental feature for now.
+
+

plugins/woocommerce/src/Internal/Features/FeaturesController.php

@@ -116,13 +116,16 @@ class FeaturesController {
 			// Options for HPOS features are added in CustomOrdersTableController to keep the logic in same place.
 			'custom_order_tables'  => array( // This exists for back-compat only, otherwise it's value is superseded by $hpos_authoritative option.
 				'name'               => __( 'High-Performance order storage (COT)', 'woocommerce' ),
+				'is_experimental'    => true,
 				'enabled_by_default' => false,
 			),
 			$hpos_authoritative    => array(
-				'name' => __( 'High performance order storage', 'woocommerce' ),
+				'name'            => __( 'High performance order storage', 'woocommerce' ),
+				'is_experimental' => true,
 			),
 			$hpos_enable_sync      => array(
-				'name' => '',
+				'name'            => '',
+				'is_experimental' => true,
 			),
 			'cart_checkout_blocks' => array(
 				'name'            => __( 'Cart & Checkout Blocks', 'woocommerce' ),