Change the identifier of the extension point used to make the HTML head plugable

In the frame of XWIKI-16093, it was agreed on that the identifier of the extension point introduced to make the HTML head plugable would be "org.xwiki.platform.html.head". However, until now, it remained declared as "org.xwiki.platform.head" instead. This issue is about fixing this inconsistency. It is created as a distinct one from XWIKI-16093 since that one was marked as fixed in 11.1 RC1, which was released in the meantime.