Bump lit-element from 2.2.1 to 2.3.1

Bumps [lit-element](https://github.com/Polymer/lit-element) from 2.2.1 to 2.3.1.
- [Release notes](https://github.com/Polymer/lit-element/releases)
- [Changelog](https://github.com/Polymer/lit-element/blob/master/CHANGELOG.md)
- [Commits](lit/lit-element@v2.2.1...v2.3.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package-lock.json

@@ -38,7 +38,7 @@
   },
   "dependencies": {
     "@rollup/plugin-node-resolve": "^7.1.1",
-    "lit-element": "^2.2.1",
+    "lit-element": "^2.3.1",
     "moment": "^2.24.0",
     "node-google-calendar": "^1.1.1"
   },