PP-6147: Upgrade node to v12.4.1

This is the latest version of node supported by GOV.UK PaaS. We need to
be on at least v12.9.0 to use AWS CloudFront's Field Level Encryption.

The issue with `statsd` we encountered when we upgraded to node v12.13.1
(see [PP-5917](https://payments-platform.atlassian.net/browse/PP-5917))
looks like it's fixed in v12.4.1 (according to [the node release notes](https://github.com/nodejs/node/blob/master/doc/changelogs/CHANGELOG_V12.md#2020-01-07-version-12141-erbium-lts-bethgriggs)).

**NOTE**: I'm not sure how to test that the statsd issue hasn't
regressed, so this will need a look by someone with more understanding
of the issue.

The hash used in the Dockerfiles was obtained with:

```
$ docker pull node:12.14.1-alpine
12.14.1-alpine: Pulling from library/node
c9b1b535fdd9: Pull complete
32eb17722c57: Pull complete
c6a4a31e8940: Pull complete
3f01d5292e29: Pull complete
Digest: sha256:1dd4309479f031295f3dfb61cf3afc3efeb1a991b012e105d1a95efc038b72f6
Status: Downloaded newer image for node:12.14.1-alpine
docker.io/library/node:12.14.1-alpine
```

.nvmrc

@@ -1 +1 @@
-12.2.0
+12.14.1

Dockerfile

@@ -1,4 +1,4 @@
-FROM node@sha256:2ab3d9a1bac67c9b4202b774664adaa94d2f1e426d8d28e07bf8979df61c8694
+FROM node@sha256:1dd4309479f031295f3dfb61cf3afc3efeb1a991b012e105d1a95efc038b72f6
 
 ### Needed to run appmetrics and pact-mock-service
 COPY sgerrand.rsa.pub /etc/apk/keys/sgerrand.rsa.pub

docker/build_and_test.Dockerfile

@@ -1,4 +1,4 @@
-FROM node@sha256:2ab3d9a1bac67c9b4202b774664adaa94d2f1e426d8d28e07bf8979df61c8694
+FROM node@sha256:1dd4309479f031295f3dfb61cf3afc3efeb1a991b012e105d1a95efc038b72f6
 
 ### Needed to run appmetrics and pact-mock-service
 COPY sgerrand.rsa.pub /etc/apk/keys/sgerrand.rsa.pub

package.json

@@ -5,7 +5,7 @@
   "private": true,
   "license": "MIT",
   "engines": {
-    "node": "^12.11.1"
+    "node": "^12.14.1"
   },
   "standard": {
     "globals": [