Restore obsolete l3_server port role for backward compatibility

[chrismarget-j]

#428 eliminated the l3_server Port Role which existed in the API but has been removed from the Apstra GUI ages ago.

It has just occurred to me that we need to phase this out more carefully:

If a user has been setting the value, they're going to encounter it during Read(), which will lead to an error.

We need to restore this string to the SDK (for now) and modify the terraform provider to ensure the string is never sent to Apstra.