Test/Enable Node.js 14 support for SQL 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:

SQL connectors:

• loopback-connector
• loopback-connector-mysql
• loopback-connector-postgresql
• loopback-connector-mssql
• loopback-connector-oracle
• loopback-connector-db2 (need to find out if ibm_db supports Node.js 14)

[dhmlau]

loopback-connector: PR loopbackio/loopback-connector#180.

[dhmlau]

We've enabled Node.js 14 in Jenkins, so it should be part of the CI automatically.
For mssql connector, CI all passes. See loopbackio/loopback-connector-mssql#233

[dhmlau]

loopback-connector-db2 has been verified as well.
See loopbackio/loopback-connector-db2#152

[dhmlau]

What's left is to test loopback-connector-oracle works on Node.js 14. I believe currently there's some CI issue (not related to Node.js 14) in that repo.