Clean up schema for table ci_runners

From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Wrong data type: contacted_at should be timestamptz
• Wrong data type: token should be bytea
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL
• Duplicate data: name, version, revision, platform, architecture all are repeated throughout, separate table with integers would be more efficient
  • This may be tricky if the data is arbitrary and provided by runners. Storing this in a table would require a block of code that does an INSERT or SELECT depending on whether the data already exists or not. This is quite complex, so do this last.