(docs) add release notes for 7.20.0

Author: austb

documentation/release_notes_7.markdown

@@ -15,6 +15,20 @@ canonical: "/puppetdb/latest/release_notes.html"
 
 # PuppetDB: Release notes
 
+## PuppetDB 7.20.0
+
+Released October 22 2024
+
+### Improvements
+
+* Released support and packages for Debian 12 (bookworm)
+
+### Bug fixes
+
+* Added a database constraint to prevent duplicate catalogs. If your database
+  contains any duplicate catalogs, only the most recent catalog for each
+  certname will be kept.
+
 ## PuppetDB 7.19.1
 
 Released July 23 2024