chore(deps): update dependency semantic-release to v21

Author: renovate[bot]

package.json

@@ -37,7 +37,7 @@
     "istanbul": "0.4.5",
     "mocha": "6.2.3",
     "opener": "1.5.2",
-    "semantic-release": "15.14.0",
+    "semantic-release": "21.0.0",
     "sinon": "7.5.0"
   }
 }