refactor: mods mappings now in Cocina::Models::Mapping module

[ndushay]

NOTE: Changes to openapi.yml require updating openapi.yml for sdr-api and dor-services-app and generating models - see README.

Why was this change made? 🤔

as discussed with team, we decided to

• put the mods mappings into the Cocina::Models::Mapping module
• rename them FromMods and ToMods (instead of FromFedora and ToFedora)
• move Purl to Cocina::Models::Mapping level (was in "FromFedora")
• upgrade rubocop and rubocop-rspec gems (was necessary to get rubocop to pass)

Part of #431

No openapi.yml was harmed in the creation of this PR.

How was this change tested? 🤨

rspec mapping tests (and all other rspec tests) pass.

⚡ ⚠ If this change has cross service impact, run integration tests and/or test in [stage|qa] environment, in addition to specs. ⚡