Update RELEASE_NOTES.md for 1.5.20 release (#265)

* Update RELEASE_NOTES.md for 1.5.20 release

* Minor formatting fix

Author: Arkatufus

RELEASE_NOTES.md

@@ -1,3 +1,13 @@
+### 1.5.20 May 11th 2024 ###
+
+* [Update Akka.NET to v1.5.20](https://github.com/akkadotnet/akka.net/releases/tag/1.5.20)
+* [Update Akka.Hosting to 1.5.20](https://github.com/akkadotnet/Akka.Hosting/releases/tag/1.5.20)
+* [Update Npgsql minimum version to v8.0.3](https://github.com/akkadotnet/Akka.Persistence.PostgreSql/pull/261)
+
+**PostgreSQL minimum version update**
+
+The minimum version of the Npgsql NuGet package have been upgraded to 8.0.3 due to [this CVE report](https://github.com/advisories/GHSA-x9vc-6hfv-hg8c)
+
 ### 1.5.15.1 January 29 2024 ###
 
 * Fix Npgsql upper range version number