Deprecate the P3P plugin and set it to disabled for new installations.

Based on discussion in PR #4678 about the obsolte P3P Policy plugin this PR deprecates the plugin and disables it for new installations.

See detailed explanation in the other PR but the gist is that this headers are only supported by IE6 and since we officially only support IE8+ it doesn't make sense to keep the plugin in eternity. Thus it will be removed with Joomla 4.0 according to our current strategy.