Use Jetpack_RelatedPosts::get_options() to get options for server-rendered Related Posts

[westonruter]

It was discovered by @jamesozzie that filtering options for rendering Related Posts does not work on AMP pages. For example, in response to a support forum topic he was looking to reduce the number of posts displayed from 3 to 2 with code like so:

function jetpackme_more_related_posts( $options ) {
    if ( function_exists('is_amp_endpoint') && is_amp_endpoint()) {
        $options['size'] = 2; //change the number for the amount required
    }
    return $options;
 }
add_filter( 'jetpack_relatedposts_filter_options', 'jetpackme_more_related_posts' );

This turned out not to work because AMP pages render the Related Posts using Jetpack_RelatedPosts::get_server_rendered_html() and this method was calling Jetpack_Options::get_option( 'relatedposts', array() ) rather than Jetpack_RelatedPosts::get_options(), and only the later method applies the jetpack_relatedposts_filter_options filter (as well as apply other normalizations).

This PR updates \Jetpack_RelatedPosts::get_server_rendered_html() to use Jetpack_RelatedPosts::get_options() rather than the lower-level Jetpack_Options::get_option( 'relatedposts', array() ).

Issue introduced in Jetpack 7.6 via #13028 for #9556.

Changes proposed in this Pull Request:

• Ensure options for rendering Related Posts are filterable when rendering on AMP pages.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• If you're an Automattician, include a shortlink to the p2 discussion with Jetpack Product here.

Testing instructions:

• Add a jetpackme_more_related_posts filter to change the size (see above).
• Verify the size change is applied to AMP and non-AMP pages alike.

Proposed changelog entry for your changes:

• Ensure options for rendering Related Posts are filterable when rendering on AMP pages. Use Jetpack_RelatedPosts::get_options() to get options for server-rendered Related Posts

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review

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[gravityrail]

Good catch! LGTM

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello westonruter! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D37883-code before merging this PR. Thank you!