Ignore file_md5 column in VirtualRegistries::Container::Cache::Entry

What does this MR do and why?

This MR implements Release M of the three-release process to remove the file_md5 column from the virtual_registries_container_cache_entries table.

Following the guidelines in Avoiding downtime in migrations - Ignoring the column (Release M).

Why remove file_md5?

The file_md5 column is not needed - we use file_sha1 for container images.

Changes in this MR (Release M)

• Add ignore_column :file_md5 directive to the VirtualRegistries::Container::Cache::Entry model
• Remove sha_attribute :file_md5 declaration
• Remove validation for file_md5
• Remove all code references to file_md5:
  • VirtualRegistries::Container::Cache::Entries::CreateOrUpdateService
  • VirtualRegistries::Container::HandleFileRequestService
  • API entity API::Entities::VirtualRegistries::Container::Cache::Entry
• Update factory and specs to remove file_md5 references

Next steps

• Release M+1 (18.8): Drop the file_md5 column from the database
• Release M+2 (18.9): Remove the ignore_column directive

Related

Related to !205956 (merged) (comment 2790898109)

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