Rename duo_self_hosted AddOn to self_hosted_dap

What does this MR do and why?

Renames the duo_self_hosted AddOn to duo_self_hosted_dap to eliminate confusion with Self-Hosted Models for Duo Enterprise, which is a different offering. The new name clarifies that this AddOn is specifically for the Self-Hosted Duo Agent Platform (DAP).

Changes

• Renamed the AddOn constant from DUO_SELF_HOSTED to DUO_SELF_HOSTED_DAP in constants and models
• Updated all references in service classes, scopes, and factory traits
• Updated user-facing strings to "GitLab Self-Hosted Duo Agent Platform"
• Created database migration to rename existing duo_self_hosted entries to duo_self_hosted_dap in subscription_user_add_on_assignment_versions
• Updated all tests to use the new naming convention

How to test

This is a rename, so everything should keep working as expected. This code is not in use yet, since we do not sell this AddOn as of now. So there was no (official) way to access this code previously, clicking through some of the Duo pages should not throw any errors though

Migration

Since we do not provision this AddOn yet, we might not need a migration

[ gprd ] production> GitlabSubscriptions::UserAddOnAssignmentVersion.distinct.pluck(:add_on_name)
=> ["code_suggestions", "duo_core", "duo_enterprise"]

Related to

Closes #583583