Correct link to migration docs in patch blog template

Reuben Pereirarequested to merge
rp/update-patch-template into master

What does this MR do?

The template was updated in gitlab-com/www-gitlab-com!99679 (merged). Since the actual template now lives in this repo, we need to update this template file.

I've opened an MR to remove the template from the www-gitlab-com repo: gitlab-com/www-gitlab-com!97149 (merged).

Author Check-list

  • Has documentation been updated?

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