Hide `GTIN` field for new installs make readonly for existing installs #3362

Job	Run time
Get WP and WC version Matrix	2s
PHP unit tests (for Release Candidates)	0s
PHP unit tests - PHP 8.2, WP latest, WC 9.3.3	3m 25s
PHP unit tests - PHP 8.2, WP latest, WC 9.2.3	1m 33s
PHP unit tests - PHP 8.2, WP latest, WC 9.1.4	1m 29s
PHP unit tests - PHP 8.3, WP latest, WC latest	1m 21s
PHP unit tests - PHP 7.4, WP 6.4.5, WC 9.1.4	1m 49s
	9m 39s

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