Add start of announcement blog

Author: dondonz

blog/2023-07-11-v21-released.md

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+---
+title: "Version 21 released"
+authors: donna
+slug: v21-released
+---
+
+We are pleased to announce the release of graphql-java v21.0! Thanks to everyone in the community who contributed to the release, whether that was code, helping to report issues, or participating in discussions.
+
+And a very Happy 8th Birthday to graphql-java, who celebrated their birthday last week!
+
+## Breaking change release: new Java 11 minimum
+This is a breaking change release. This is the first release of graphql-java to use Java 11.
+
+TODO: add other release highlights
+
+See the full release notes on [GitHub](TODO: add link)