[11.x] Do not overwrite existing link header(s) in `AddLinkHeadersForPreloadedAssets` middleware #42054

Job	Run time
PHP 8.2 - PHPUnit 10.5 - prefer-lowest	2m 25s
PHP 8.2 - PHPUnit 10.5 - prefer-stable	2m 25s
PHP 8.2 - PHPUnit 11.0.1 - prefer-lowest	2m 26s
PHP 8.2 - PHPUnit 11.0.1 - prefer-stable	2m 27s
PHP 8.3 - PHPUnit 10.5 - prefer-lowest	2m 35s
PHP 8.3 - PHPUnit 10.5 - prefer-stable	1m 59s
PHP 8.2 - PHPUnit 10.5 - prefer-lowest - Windows	2m 24s
PHP 8.3 - PHPUnit 11.0.1 - prefer-lowest	2m 10s
PHP 8.2 - PHPUnit 10.5 - prefer-stable - Windows	2m 22s
PHP 8.3 - PHPUnit 11.0.1 - prefer-stable	3m 14s
PHP 8.2 - PHPUnit 11.0.1 - prefer-lowest - Windows	2m 24s
PHP 8.2 - PHPUnit 11.0.1 - prefer-stable - Windows	2m 19s
PHP 8.3 - PHPUnit 10.5 - prefer-lowest - Windows	2m 17s
PHP 8.3 - PHPUnit 10.5 - prefer-stable - Windows	2m 14s
PHP 8.3 - PHPUnit 11.0.1 - prefer-lowest - Windows	2m 21s
PHP 8.3 - PHPUnit 11.0.1 - prefer-stable - Windows	2m 21s
	38m 23s

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