Header images should use get_header_image_tag()

[raamdev]

Currently, the <img> tag for the header image is built manually in independent_publisher_posted_author_card():

https://github.com/raamdev/independent-publisher/blob/36dd2c2856221850c4a5491f18193a325c82e810/inc/template-tags.php#L491

It would be better to use get_header_image_tag() to build this tag, as that will include the srcset attributes for responsive images automatically.