Hi Heather!
Installing the Search Index Purge Plugin will do the following:
– Every attachment URL will return a 410 status code.
– A static XML sitemap, containing all the attachment URLs on a given site will be created. The post modified date for each of those URLs is the activation date and time of the plugin.
The XML sitemap with the recent post modified date will make sure that Google spiders all those URLs again. The 410 status code will make sure Google takes them out of its search results in the fastest way possible. After six months the attachment URLs should be gone from the search results.
As for your HTTP and HTTP sitemaps, a correct SSL migration should result in a redirect for each existing HTTP link of your website to its HTTPS equivalent. So there shouldn’t be an HTTP sitemap anymore. These redirects should be done at the server level and the Yoast SEO plugin is unable to create such type of redirects. You need to get in touch with your web host for assistance.
For more information on HTTP to HTTPS migrations, we recommend the following guides:
https://yoast.com/dev-blog/move-website-https-ssl/
https://kinsta.com/blog/http-to-https/