doc: changelog 4.1.21

Author: MrSrsen

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * [c41a0bca4](https://github.com/api-platform/core/commit/c41a0bca49778663743a52e9da9ddb3b1439c2f8) fix(jsonld): child class @type shortName (#7312)
 * [d26088ba1](https://github.com/api-platform/core/commit/d26088ba1080f9fb8d94db5b476b0322d2a14ab2) chore: allow doctrine-persistence:^4.0 (#7309)
+* [e0754a2f](https://github.com/api-platform/core/commit/e0754a2f606e6279269722ef81b4b11fc927d836) fix(openapi): nullable default values in operation openapi definition
 
 ## v4.1.19