Test/Enable Node.js 12 for service connectors

[dhmlau]

Description / Steps to reproduce / Feature proposal

Originated from: #3072

Acceptance Criteria

For the following service connectors, test locally on Node.js 12 and see if all tests passed.

If yes, add Node.js 12 to CI
If not, try to fix it for simple fixes. If you don't think you can fix it in a day, open a new issue to keep track of it.

• loopback-connector-remote -> Drop support for Node.js 6 and add Node.js 12 to CI  strongloop/loopback-connector-remote#97
• loopback-connector-rest -> Drop Node.js 6 support and add Node.js 12 to CI loopback-connector-rest#140
• loopback-connector-soap -> Drop Node.js 6 support and add Node.js 12 to CI loopback-connector-soap#109
• [ ] loopback-connector-swagger deprecated
• loopback-connector-jsonrpc -> drop node 6 and add node 12 to CI strongloop/loopback-connector-jsonrpc#25
• loopback-connector-openapi -> add node 12 to CI loopback-connector-openapi#4

[nabdelgadir]

Closing this as done.