doc(mail): add initial release note for deprecated mail config #7424

doc/release-notes/7424-mailsession.md

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+## New way to configure mail transfer agent
+
+With this release, we deprecate the usage of `asadmin create-javamail-resource` to configure your MTA.
+Instead, we provide the ability to configure your SMTP mail host using JVM options with the flexibility of MicroProfile Config.
+
+At this point, no action is required if you want to keep your current configuration.
+Warnings will show in your server logs to inform and remind you about the deprecation.
+A future major release of Dataverse may remove this way of configuration.