Versioning migration fixups

[finlay-jisc]

The purpose of this PR was to fix some issues with the versioning migration script that are preventing us from running it properly on AWS.

There were some basic formatting changes to the prisma schema and versioning migration guide.

I've followed through the steps in the updated guide locally to confirm that they still work. In a slight change to the process I've included the script in the same commit as the first DB migration, so that's now run before checking out the second commit.

The seed script still runs OK after recreating the DB migrations.

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Acceptance Criteria:

• Clean up any changes from previous attempts of running the script so that we don't always have to restore the database after a failed attempt
• Don't run lots of database tasks in parallel because it will use up the available connection pools. Instead, run them one after another

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Checklist:

• Local manual testing conducted
• Automated tests added
• Documentation updated