Improve Singleton instantiation in SchedulerRepository

Refactored the Singleton pattern in SchedulerRepository to use the Bill Pugh Singleton Holder pattern for lazy initialization and thread safety without explicit synchronization.

This PR...

Fixes issue #

Changes

Refactored the Singleton pattern in SchedulerRepository

Checklist

[x] tested locally
[ ] updated the docs
[ ] added appropriate test
[x] signed-off on the DCO referenced in the CONTRIBUTING link below via git commit -s on my commits, and submit this code under terms of the Apache 2.0 license and assign copyright to the Quartz project owners (If you're not using command-line, you can use a browser extension )

In submitting this contribution, I agree to the terms of contributing as referred to here: https://github.com/quartz-scheduler/contributing/blob/main/CONTRIBUTING.md

quartz-scheduler / quartz

Improve Singleton instantiation in SchedulerRepository #1231

Changes

Checklist

[x] signed-off on the DCO referenced in the CONTRIBUTING link below via `git commit -s` on my commits, and submit this code under terms of the Apache 2.0 license and assign copyright to the Quartz project owners (If you're not using command-line, you can use a browser extension )

quartz-scheduler / quartz

Improve Singleton instantiation in SchedulerRepository #1231

Changes

Checklist

[x] signed-off on the DCO referenced in the CONTRIBUTING link below via git commit -s on my commits, and submit this code under terms of the Apache 2.0 license and assign copyright to the Quartz project owners (If you're not using command-line, you can use a browser extension )

[x] signed-off on the DCO referenced in the CONTRIBUTING link below via `git commit -s` on my commits, and submit this code under terms of the Apache 2.0 license and assign copyright to the Quartz project owners (If you're not using command-line, you can use a browser extension )