Test/Enable Node.js 14 support for SQL connectors

@jannyHou がすでに取り組んでいます。

2020年10月27日 から。

Originated from epic: https://github.com/strongloop/loopback-next/issues/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:_
- [x] loopback-connector
- [x] loopback-connector-mysql
- [x] loopback-connector-postgresql
- [x] loopback-connector-mssql
- [ ] loopback-connector-oracle
- [x] loopback-connector-db2 (need to find out if ibm_db supports Node.js 14)