Get pg as a dependency of metasploit_data_models

[#38274165]

metasploit_data_models already declares pg as a runtime dependency in
its gemspec, so there is no need to add pg as a direct dependency of
metasploit-framework, since metasploit-framework only needs pg for
metasploit_data_models.
Remove cred_files migration

[#41837027]

Mdm::CredFile is only used in Pro, so for metasploit_data_models 0.4.0,
Mdm::CredFiles has been moved to Pro, so the migration has been moved to
Pro too.
Merge branch 'bug/pro-only-models'

Updates to use MDM 0.4.0 (was using 0.3.0)