Test/Enable Node.js 14 support for service connectors

[dhmlau]

Originated from epic: #6019

Acceptance Criteria

For the following connectors, test locally on Node.js 14 and see if all tests passed.

• If yes, add Node.js 14 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.

Here is the list of connectors:

• loopback-connector-remote
• loopback-connector-rest
• loopback-connector-soap
• loopback-connector-jsonrpc
• loopback-connector-openapi

[dhmlau]

Closing as done.