doc: Add release note

doc/release-notes-27213.md

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+P2P and network changes
+------
+
+- Nodes with multiple reachable networks will actively try to have at least one
+  outbound connection to each network. This improves individual resistance to
+  eclipse attacks and network level resistance to partition attacks. Users no
+  longer need to perform active measures to ensure being connected to multiple
+  enabled networks.