Self-explanatory, the updated bundles for NeoForge 1.21.1 require Collective 8.4, but no supported 8.4 version is available for NeoForge 1.21.1 (Only available for 1.21.6-8).

[14:22:32.225] [main/ERROR] [loading.ModSorter/LOADING]: Missing or unsupported mandatory dependencies:
Mod ID: 'collective', Requested by: 'serilumscustomizationbundle', Expected range: '[8.4,)', Actual version: '8.3'
Mod ID: 'collective', Requested by: 'serilumsrpgbundle', Expected range: '[8.4,)', Actual version: '8.3'

Specifically, these are the updated bundles that require Collective 8.4:

• serilumscustomizationbundle-1.21.1-2.6.jar
• serilumsrpgbundle-1.21.1-2.5.jar

The older versions work fine (require Collective 8.3, which is available for NeoForge 1.21.1):

• serilumscustomizationbundle-1.21.1-2.5.jar
• serilumsrpgbundle-1.21.1-2.4.jar