Upgrade the minimal XWiki version to 15.10

Raise the minimal XWiki version supported by the Markdown syntaxes from XWiki 12.10 to XWiki 15.10, by using the latest 15.10.x contrib parent (parent-rendering 15.10.15).

Consequences:

• The extension can no longer be installed on XWiki versions older than 15.10.
• When parsing CommonMark Markdown 1.2, images now get an id (e.g. Iimage.png), generated the same way as by the XWiki Syntax parsers since XWiki 14.2, with ids unique across headings and images of the document. This allows images to be targeted by anchors.
• The Java/Spoon/release overrides that were needed with the 12.10 parent are removed since the 15.10 parent provides them.