docs: Add 1.3.1 release note

docs/release-notes/release-notes.1.3.1.md

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+# vspd 1.3.1
+
+This is a patch release of vspd which includes the following changes:
+
+- webapi: Add missed tickets to admin page ([#451](https://github.com/decred/vspd/pull/451)).
+
+Please read the [vspd 1.3.0 release notes](https://github.com/decred/vspd/releases/tag/release-v1.3.0)
+for a full list of changes since vspd 1.2.
+
+## Dependencies
+
+vspd 1.3.1 must be built with go 1.20 or later, and requires:
+
+- dcrd 1.8.0
+- dcrwallet 1.8.0
+
+When deploying vspd to production, always use release versions of all binaries.
+Neither vspd nor its dependencies should be built from master when handling
+mainnet tickets.