Better error message for unsupported 24.04 base

[yanksyoon]

What needs to get done

Create a model validator when using the to be deprecated bases model that raises helpful message.

charmcraft/charmcraft/models/project.py

Line 492 in ff00a89

raise errors.CraftValidationError(

Why it needs to get done

Currently the charmcraft.yaml, when using model.bases with 24.04, will error with

Bad charmcraft.yaml content:                                                                   
- base requires 'platforms' definition: {'name': 'ubuntu', 'channel': '24.04', 'architectures': ['amd64']} (in field 'bases[2]')    

messaage.
This is not a good UX. Many people have been raising duplicate questions regarding this cryptic errior on the Charmcraft matrix channel.