release: switch the next branch to v21.0.0-next.0

Author: crisbeto

package.json

@@ -52,7 +52,7 @@
     "ci-docs-monitor-test": "node --no-warnings=ExperimentalWarning --loader ts-node/esm/transpile-only scripts/docs-deploy/monitoring/ci-test.mts",
     "prepare": "husky"
   },
-  "version": "20.3.0-next.0",
+  "version": "21.0.0-next.0",
   "dependencies": {
     "@angular-devkit/core": "catalog:",
     "@angular-devkit/schematics": "catalog:",