Update post migration to drop column only if it exists

What does this MR do and why?

This MR updates a post migration so that columns are dropped only when they exist.

Without this change, upgrades from 16.4 to 16.5 are failing on migrations, please see original report here: #429017 (closed)

Describe in detail what merge request is being backported and why

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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