Update whole Gantry5 to be native Joomla 5

[peter1szalatnay]

Hi Gantry Team,

I put the effort in to convert Gantry5 5.5 to be native Joomla 5 implementation. I have dropped support for Joomla 3.x and 4.x in this release.

I have done extensive testing to see that the functionality should be the same as before.

The version in Changelog has been updated to version 5.6 and the update xml list should reflect that it only supports Joomla 5.x versions.

Cheers

[peter1szalatnay]

Found a bug in administrator/components/com_gantry5/templates/partials/updates.html.twig

need to update the code to reflect the changes for quickicon update

[N8Solutions]

Tagging @rhukster & @hexplor to get them to have a look at this and weigh in to see if they'll approve it.

I will try it out as soon as possible and report back. I encourage others to do the same.

Thanks for doing this @peter1szalatnay!