Update principle 3 to make upper case prefix mandatory (#2605)

* Update principle 3 to make upper case prefix mandatory

For a while it was not clear whether upper case prefixes are mandated or merely encouraged for new submissions. 

We have conducted a vote (#2596) which clarifies the situation. The decision was that for new ontology submissions, ontology prefixes _must be_ mandatory.

* Update principles/fp-003-uris.md

Co-authored-by: Darren A. Natale <nataled@users.noreply.github.com>

* Update principles/fp-003-uris.md

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Co-authored-by: Darren A. Natale <nataled@users.noreply.github.com>

Author: matentzn

principles/fp-003-uris.md

@@ -21,7 +21,10 @@ Each ontology MUST have a unique IRI in the form of an OBO Foundry permanent URL
 
 ### Ontology Namespace:
 
-The namespace is a short string of letters (usually 2-5) that represents the ontology. When used as part of a CURIE, on its own, or as part of a term ID, the namespace is generally capitalized. When used as part of the ontology IRI, the namespace is in lower case. See examples below.
+The namespace is a short string of letters (usually 2-5) that represents the ontology. 
+When used as part of a CURIE, on its own, or as part of a term ID, the namespace MUST be capitalized.
+As per [vote on 31st May 2024](https://github.com/OBOFoundry/OBOFoundry.github.io/issues/2596), this applies to all ontologies submitted for acceptance after the 1st June 2024. Ontologies submitted before the 1st June 2024 may retain their mixed case prefixes.
+When used as part of the ontology IRI, the namespace is in lower case. See examples below.
 
 To request a new namespace, ontology developers MUST follow the guidelines outlined here. Note that very short namespaces (2-3 characters) are reserved for ontologies that cover a general domain and are likely to be frequently used.