Fix bug parsing memory_limit in product importer (woocommerce#46051)

Previously, the code assumed that all memory_limit values used the "M"
suffix (for megabytes); values using "G" (for gigabytes), "K" (for
kilobytes), or no suffix (for bytes) were all parsed incorrectly.  Now,
all possible values should be handled correctly.

This bug has actually been fixed twice before in other locations:

Commit 5384033 (woocommerce#30908)

Commit 4557108 (woocommerce#21557)

plugins/woocommerce/changelog/do-not-assume-megabytes

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Fix bug parsing memory_limit in product importer

plugins/woocommerce/includes/import/abstract-wc-product-importer.php

@@ -747,7 +747,7 @@ protected function get_memory_limit() {
 			// Unlimited, set to 32GB.
 			$memory_limit = '32000M';
 		}
-		return intval( $memory_limit ) * 1024 * 1024;
+		return wp_convert_hr_to_bytes( $memory_limit );
 	}
 
 	/**