Bump minimum node version to 16. (#316)

* Bump minimum node version to 16.

We already made the mistake of using features from 16 without realising
Previously: https://github.com/matrix-org/mjolnir/pull/192

* Make sure CI uses specific version of node and we also lint.

https://github.com/matrix-org/pipelines/blob/master/mjolnir/pipeline.yml#L13

.github/workflows/mjolnir.yml

@@ -11,11 +11,38 @@ env:
 
 jobs:
   build:
+    name: Build & Lint
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Specifically use node 16 like in the readme. 
+      uses: actions/setup-node@v3
+      with:
+        node-version: '16'
+    - run: yarn install
+    - run: yarn build
+    - run: yarn lint
+  unit:
+    name: Unit tests
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Specifically use node 16 like in the readme. 
+      uses: actions/setup-node@v3
+      with:
+        node-version: '16'
+    - run: yarn install
+    - run: yarn test
+  integration:
     name: Integration tests
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
+    - uses: actions/setup-node@v3
+      with:
+        node-version: '16'
     - name: install mx-tester
       run: cargo install mx-tester
     - name: Setup image