rocket_lrc_processed_tags should be case insensitive

[MathieuLamiot]

Context

As reported by the support team while testing, elements passed to the filter rocket_lrc_processed_tags (renaming ongoing, could still be named rocket_lazy_render_content_processed_tags) must be added as uppercase currently.
On OCI feature, we are agnostic of upper/lower case so we should align.

The root cause and possible fix are already discussed here: https://wp-media.slack.com/archives/C07LKLTKBG8/p1725516717052949

Expected behavior

Elements added to the processed_tags must be handled correctly whether they are added to the filtered array as lower or upper case.

Acceptance criteria

On a page with a h1 element that should get the LRC hash (depth being properly configured):

• When using the following snippet, the hash is added to the h1 element:

add_filter( 'rocket_lazy_render_content_processed_tags', function( $tags ){
	$tags[]= "H2";
	$tags[]= "H1";
    return $tags;
} )

• When using the following snippet, the hash is added to the h1 element:

add_filter( 'rocket_lazy_render_content_processed_tags', function( $tags ){
	$tags[]= "h2";
	$tags[]= "h1";
    return $tags;
} )

[MathieuLamiot]

Grooming: https://wp-media.slack.com/archives/C07LKLTKBG8/p1725520868703359?thread_ts=1725516717.052949&cid=C07LKLTKBG8