Add UDF v2 for 4.2.1 on the stable channel

That's needed in order to run the automated test suite on the 4.2.1
images.

wiki/src/upgrade/v2/Tails/4.2.1/amd64/stable/upgrades.yml

+---
+build-target: amd64
+channel: stable
+initial-install-version: 4.2.1
+product-name: Tails

wiki/src/upgrade/v2/Tails/4.2.1/amd64/stable/upgrades.yml.pgp

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+
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+=LQpK
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