Install Vim dependency for assembly script

Why is this change being made?

We have been working towards greater automation of the Release Post. Towards that goal we have a few scheduled pipelines that kickoff the process, assemble the script and update deprecations.

The Release Post Content Assembly scheduled pipeline runs at 08:00 UTC on day-of-month 18. However, since it's inception, it fails in the pipeline environment, but succeeds in the local environment.

An example failure: https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/4134161178

bundler: failed to load command: bin/release-post-build (bin/release-post-build)
/builds/gitlab-com/www-gitlab-com/lib/release_posts/helpers.rb:23:in `popen': No such file or directory - ex (Errno::ENOENT)

The failure is in the replace method where we make a system call to ex editor (often installed alongside vim) for some crafty substitution.

It turns out that the default image, a variant of Debian Bullseye does not have Vim installed by default.

Before updating the base image for all other steps, we're proposing to install Vim in a before_script configuration step.

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