Upgrade to Elastic search client 8.15.2

As we already discussed (Thomas mentioned it again in a chat), the solution is to either develop a small standalone app (or better use an apache web server mod if that's possible) mapped to extensions.xwiki.org/activeinstalls2 and that will perform conversions of the formats sent by XWiki instances (we have the xwiki version in the request BTW so we can support different formats depending on the XWiki version too) and then forward the converted format to ES. We should probably support the current ES format used by Active installs 2 and consider it the XWiki format (that allows for backward compatibility and possibly still allow us to use the parsers coded by ES so that we don't have to redo that part - not sure about this part, depends if we can stay on the same version of the ES client library or not).

Note that even if we could do this on the XWiki instance for xwiki.org we should probably avoid putting more load on it (right now this would mean more than 10K requests every day: all the active installs + all the installs that don't last more than 1 day. It's quite low, since it means around 7-10 requests more per minute but still, that adds up) and more importantly it would be a lot slower than it should be (an XWiki request takes a lot more time than, say, an apache web server request).