fix: implement CronJon type generics

[nerdstep]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• [ ] Tests for the changes have been added (for bug fixes / features)
  • N/A since this change is for types only
• [ ] Docs have been added / updated (for bug fixes / features)
  • N/A

PR Type

What kind of change does this PR introduce?

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior?

Issue Number: #1537

What is the new behavior?

Introduced missing type generics for CronJob.

Does this PR introduce a breaking change?

• [ ] Yes
• [X] No

Other information

[alexyang-e1]

Hi team, any chance we could merge this in please? TIA 🙏