Use SPDX identifier in license field of META6.json

[samcv]

Use the standardized identifier for the license field.
For more details see https://design.perl6.org/S22.html#license

[slobo]

Thanks @samcv. I think I ripped some parts right out of Xorg header files, do you think maybe MIT would be a better license choice?
I don't have any personal preference, other than to play fair...

Also, I think the examples are based on code from different licenses, but don't think that matters for META6...

[samcv]

I would do Artistic-2.0 AND X11 since there's an X11 specific SPDX-identifier. Thanks for the honesty :)

SPDX spec uses OR if it's dual licensed, and uses AND if the project has sections which fall under separate licenses

[slobo]

thanks @samcv, sorry for the long delay in merging this.