Make utf8mb4 the default in XWiki Debian packages

This is about things like:

• the generated hibernate.cfg.xml
• the Debian package

Note that there is several downside about utf8mb4:

• bigger databases
• more importantly the InnoDB storage engine has a maximum index length of 767 bytes, so for utf8 or utf8mb4 columns, you can index a maximum of 255 or 191 characters, respectively. Problem is that most of our indexed string fields are currently been designed with the 255 character limit in mind...