Draft: Drop file_md5 column from VirtualRegistries::Container::Cache::Entry

What does this MR do and why?

This MR implements Release M+1 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 - Dropping the column (Release M+1).

Why remove file_md5?

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

Changes in this MR (Release M+1)

• Drop the file_md5 column from the database in a post-deployment migration

Previous steps

• Release M (18.7): Ignored the column and removed all code references (!213182 (merged))

Next steps

• Release M+2 (18.9): Remove the ignore_column directive

Related

• Remove file_md5 column from virtual_registries_... (#581286)
• Ignore file_md5 column in VirtualRegistries::Co... (!213182 - merged)

MR acceptance checklist

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