Remove arbitrary 5-keyword limit from core spec

The limit of 5 keywords is a WordPress.org convention, not a
universal requirement. Other ecosystems (TYPO3, Shopify, VS Code)
have no such constraint. Individual extensions to the spec can
impose their own limits if needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jdevalk

specification.md

@@ -344,7 +344,7 @@ The description SHOULD be written in plain text, and clients MUST escape any spe
 
 The `keywords` property specifies keywords to assist users in searching for packages.
 
-Keywords MUST be a list, represented as a JSON Array. The list SHOULD NOT contain more than 5 items. Clients MAY truncate the list to 5 items.
+Keywords MUST be a list, represented as a JSON Array.
 
 Each item of the array MUST be a string.