sitemap entries in robots.txt are not fully qualified urls

[bram-atmire]

Describe the bug

Sitemaps entries in https://github.com/DSpace/dspace-angular/blob/main/src/robots.txt are not full qualified urls

To Reproduce

Steps to reproduce the behavior:

1. Visit the robots.txt of any DSpace 7 installation
2. Look at the sitemap urls and check if they are fully qualified urls, starting with https

Expected behavior

Expected: full qualified urls according to https://developers.google.com/search/docs/crawling-indexing/robots/robots_txt#sitemap

Related work

Link to any related tickets or PRs here.