[Gotthard2][Dark] Process dark constants starting the 21st pulse

Description

PSI has recommended against using the 1st 20 pulses while characterizing the detector and producing calibration constants.

This MR excludes these pulses while creating the dark constants. Also num_workers is updated to 25 after testing multiple values.

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

Reviewers

@calibration @mramilli