Provide a unique API in url-api module for getServerURL

I've posted a proposal that covers this issue in a broader design: https://forum.xwiki.org/t/generating-all-urls-through-the-resource-and-url-modules/19064 (design page: https://design.xwiki.org/xwiki/bin/view/Proposal/URLSerialization).

Instead of a new public URLFactory role, it proposes:

• a single internal ServerURLResolver in url-default, with getServerURL(wikiId) (follows the current request) and getConfiguredServerURL(wikiId) (configuration only, for emails such as reset password), replacing the two copies in oldcore (XWiki#getServerURL and XWikiServletURLFactory#getServerURL);
• implementing the url normalizer (XWIKI-17023) and adding a canonicalurl one on top of it;
• callers getting absolute URLs through ResourceReferenceSerializer<ResourceReference, URL> (default hint for the current request, canonical hint for configuration-only URLs), with no direct use of the server URL.

If the proposal is agreed, PR #5855 would be closed in favor of it.