LiteSpeed HTML comment appended to response breaks JSON

The WordPress AMP plugin generates JSON responses when performing AMP validation. LiteSpeed Cache is currently appending this HTML comment to all responses, including those which do not have a `text/html` content type. This is breaking the AMP plugin's ability to obtain validation results. I've had to provide users with a [workaround](https://gist.github.com/westonruter/e588bd4ed1778799b19417c838eb5e5e) plugin to disable the `litespeed_comment` filter.

This logic here:

https://github.com/litespeedtech/lscache_wp/blob/6f3e5bd70db78e1900a8ba0c68765c7257534f5c/litespeed-cache/inc/litespeed-cache.class.php#L556-L558

The condition should be changed as follows:

```php
$content_type = ini_get( 'default_mimetype' );
foreach ( headers_list() as $header ) {
list( $name, $value ) = explode( ':', $header, 2 );
if ( 'content-type' === strtolower( $name ) ) {
$content_type = trim( $value );
break;
}
}
if ( $this->footer_comment && 'text/html' === substr( $content_type, 0, 9 ) ) {
```

Support forum topics:

* https://wordpress.org/support/topic/amp-not-working-with-litespeed/#post-12716407
* https://wordpress.org/support/topic/url-validation-failed-due-to-unexpected-json-in-amp-validation-response-2/

Open litespeed-cache/inc/litespeed-cache.class.php around lines 556-558 and inspect how the footer comment is appended. Reproduce the AMP validation response or another non-HTML response, then verify that the comment is only appended for text/html content types while HTML responses retain it.

Written by the indexing model from the issue text.