lodash.get@4.4.2 is deprecated, switch to the optional chaining (?.) operator instead

**Describe the bug**
When I install this package, I receive the following deprecation warning:
```
npm warn deprecated lodash.get@4.4.2: This package is deprecated. Use the optional chaining (?.) operator instead.
```

**To Reproduce**
Install express-openapi-validator.

**Actual behavior** 
Deprecation warning is issued.

**Expected behavior**
No deprecation warnings are issued.
