update custody group count

[mehdi-aouadi]

PR Description

• The custody group count doesn't need to be updated during non finalisation
• The FirstCustodyIncompleteSlot should be update at each slot even when the custody group count doesn't change

Fixed Issue(s)

#9468
#9469

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[zilm13]

So I'd keep old code, we don't need to reset it incomplete slot when the number was decreased. But it's a good point that we should keep logging on any change.
Also I'd add tests to clarify db.setFirstCustodyIncompleteSlot( calls when oldgroupcount >, oldgroupcount <. oldgroupcount ==

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[mehdi-aouadi]

I have read the CLA Document and I hereby sign the CLA

[zilm13]

LGTM