Make migration thread count counfigurable

[gally47]

Motivation and Context

fixes #19792

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[moesterheld]

Works in general. Thank you for adding.
However, I cannot remove the value in the input field using the keyboard/mouse, I can only add to it.
Also, one small textual change.

[gally47]

Works in general. Thank you for adding. However, I cannot remove the value in the input field using the keyboard/mouse, I can only add to it. Also, one small textual change.

@moesterheld the value is not supposed to be removed you can only replace the default value (4) with any positive integer.

[moesterheld]

Works in general. Thank you for adding. However, I cannot remove the value in the input field using the keyboard/mouse, I can only add to it. Also, one small textual change.

@moesterheld the value is not supposed to be removed you can only replace the default value (4) with any positive integer.

yep. never mind, I retried it and it behaves consistently with all other numeric input fields in Graylog.
I wanted to double click the input and write another value into it, however it didn't select the value so I could directly replace it. Also, backspace won't work, which feels a little weird. I think the behavior of the numeric input could be improved in general, but it's ok for now.

[ousmaneo]

It works as expected. I have a small comment.

[ousmaneo]

We could use the getValueFromInput from the FormUtils.

[todvora]

LGTM!