Message posted by PDM on release issue contains duplicated entries

When the post-deploy migration (PDM) pipeline is executed, it posts a message on the release monthly issue listing the post-deploy migrations to be executed.

Starting with the decomposition work https://ops.gitlab.net/gitlab-com/gl-infra/db-migration/-/issues/31, the databases were split into two: main and ci, which effectively duplicated the post-deploy migrations. Now every time the PDM posts a message we have some duplicated entries, example: gitlab-org/release/tasks#4064 (comment 1026843111)

We need to adjust the release-tools logic to consider this and display the list without duplicating the entries. Note that the _n prepended at the end should also be removed.