Jetpack: Gutenberg: Update copy for Publicize for pages

[arunsathiya]

I am coming from #28681 to report this.

Steps to reproduce

• Start with a Jetpack site and have Publicize and Gutenberg enabled on it
• Create a page and write content in it
• Click on Publish to arrive at the pre-publish view
• Look at the copy on the Publicize section

What is expected

It reads Share this page and to share this page

What happens instead

It reads Share this post and to share this post

Ideally Publicize support is not available for pages, but as and when we make it available, the copy should be updated. // cc @tyxla

[tyxla]

Note: to enable Publicize support for a post type, one needs to define it specifically, like this:

add_action( 'init', '_test_jetpack_add_publicize_support_to_pages' );
function _test_jetpack_add_publicize_support_to_pages() {
    // Enable Publicize support for pages
    add_post_type_support( 'page', 'publicize' );
}

[tyxla]

Ideally, we should be flexible and adapt copy when support is enabled not only on pages, but on any post types. For that purpose, we could consume the post type labels that are available for us already.