docs: Add 1.2.1 release note.

docs/release-notes/release-notes.1.2.1.md

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+# vspd 1.2.1
+
+This is a patch release of vspd which includes the following changes:
+
+- rpc: Ignore another "duplicate tx" error ([#398](https://github.com/decred/vspd/pull/398)).
+
+## Dependencies
+
+vspd 1.2.1 must be built with go 1.19 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.