[design-system] Update linting rule for license headers

[macfarlandian]

What needs to be done? Why does it need to be done?
This package uses a linting rule that enforces the presence of a license header. Its current configuration simply checks for a text blob with a copyright date of 2020. At minimum this should be updated to 2021 ... it might be annoying for all our builds to start failing every year at midnight on January 1, so if a more flexible solution can be found, that would be preferable.

Additional context
This rule was left out of the shared configuration package to minimize disruption. However, if a good solution can be found as mentioned above, it should be adopted by the shared config package.