Downgrade Cypress to v12.

(On the understanding that v12 will continue to allow us to use
the sorry-cypress service for balancing test parallelisation.)

Reverts "Update dependency cypress to v13 (#11570)"
This reverts commit 35a6595080.

package.json

@@ -185,7 +185,7 @@
         "babel-jest": "^29.0.0",
         "blob-polyfill": "^7.0.0",
         "chokidar": "^3.5.1",
-        "cypress": "^13.0.0",
+        "cypress": "^12.0.0",
         "cypress-axe": "^1.0.0",
         "cypress-cloud": "^2.0.0-beta.0",
         "cypress-multi-reporters": "^1.6.1",