Update blog post on 15.0 breaking changes to apply correct use of the word deprecation

Why is this change being made?

Looking at https://about.gitlab.com/blog/2022/04/18/gitlab-releases-15-breaking-changes/#create, I think we are not using the word deprecation correctly here. https://en.wikipedia.org/wiki/Deprecation

I noticed that the long list of deprecations was auto-created and didn't want to rewrite every single one so I changed the formulation at the beginning.

Author Checklist

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