Bump minimum node version to >=14

Needed to merge https://github.com/matrix-org/mjolnir/pull/186 & node 12 is approaching EOL.

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:alpine
+FROM node:14-alpine
 COPY . /tmp/src
 RUN cd /tmp/src \
     && yarn install \

README.md

@@ -52,7 +52,7 @@ docker run --rm -it -v /etc/mjolnir:/data matrixdotorg/mjolnir:latest
 
 ## Build it (alternative installation)
 
-This bot requires `yarn` and Node 10.
+This bot requires `yarn` and Node 14.
 
 ```bash
 git clone https://github.com/matrix-org/mjolnir.git

package.json

@@ -39,5 +39,8 @@
     "js-yaml": "^4.1.0",
     "jsdom": "^16.6.0",
     "matrix-bot-sdk": "^0.5.19"
+  },
+  "engines": {
+    "node": ">=14.0.0"
   }
 }