Update dependency prismjs to v1.21.0 [SECURITY]

Author: renovate-bot

package.json

@@ -53,7 +53,7 @@
     "netlify-identity-widget": "1.5.4",
     "node-sass": "4.13.0",
     "nothing": "0.1.0",
-    "prismjs": "1.17.1",
+    "prismjs": "1.21.0",
     "react": "16.8.6",
     "react-dom": "16.8.6",
     "react-helmet": "5.2.1",

yarn.lock

@@ -9237,10 +9237,10 @@ pretty-error@^2.1.1:
     renderkid "^2.0.1"
     utila "~0.4"
 
-prismjs@1.17.1:
-  version "1.17.1"
-  resolved "https://registry.yarnpkg.com/prismjs/-/prismjs-1.17.1.tgz#e669fcbd4cdd873c35102881c33b14d0d68519be"
-  integrity sha512-PrEDJAFdUGbOP6xK/UsfkC5ghJsPJviKgnQOoxaDbBjwc8op68Quupwt1DeAFoG8GImPhiKXAvvsH7wDSLsu1Q==
+prismjs@1.21.0:
+  version "1.21.0"
+  resolved "https://registry.yarnpkg.com/prismjs/-/prismjs-1.21.0.tgz#36c086ec36b45319ec4218ee164c110f9fc015a3"
+  integrity sha512-uGdSIu1nk3kej2iZsLyDoJ7e9bnPzIgY0naW/HdknGj61zScaprVEVGHrPoXqI+M9sP0NDnTK2jpkvmldpuqDw==
   optionalDependencies:
     clipboard "^2.0.0"