Use online.stk for xml files

[MinIsMin]

This PR creates another variant of news.xml and asset.xml for new clients capable of using the online.stk server. See https://github.com/supertuxkart/stk-code/issues/3842

• In xmlWrite.php it simply replaces all occurences of http:addons.stk with https:online.stk. It's not the best method but there's no need to run generate*XML() twice only for changing the URL. For custom servers using this PR it replaces nothing and creates the same xml files for new clients.
• In config.EXAMPLE.php and config.example_install_for_phpstan.php I added both new const.

If this gets merged the game client needs to use the new news_v2.xml. This can be done either by changing the hardcoded URL in stk-code or rewriting the URL via .htaccess for clients using online.stk

TODO:

• Should I add and return the bytecount for the new xml files like this comment for assets2?
• Is the name 'ASSETS_V2' ok or should I reuse ASSETS2_XML_PATH if it's not used for anything?

[vampy]

Thanks. A word of advice, don't create one commit for each file modified (good thing github has the new merge and squash feature). I will try to push to production this evening.