Add validation rules for DNA Container Label

[pvangay]

Currently the guidance for DNA Container Labels states that the values must be unique and less than 20 characters. There appears to be no validation for this term because values > 20 characters or duplicate values validate just fine.

[turbomam]

Remove "<20 characters" from the guidance (implemented as a LinkML comment) or add tighter validation?

Proposed new SNTC syntax ie LinkML string_serialization: {text < 20 characters}, which would become a regular expression or LinkML pattern of ^.{1,20}$

See also linkml/linkml#176

[pkalita-lbl]

This appears to be fixed now. A plate name longer than 20 characters fails validation. Closing.