doc/release-notes: add COSMIC Beta

nixos/doc/manual/release-notes/rl-2511.section.md

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 - Syncthing has been updated to version 2.0.0.
 
+- COSMIC DE has been updated to the beta version, bringing it closer to its first stable release. This includes updates to its core components, applications, and overall stability.
+
 ## New Modules {#sec-release-25.11-new-modules}
 
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