Add missing milestone data to db/docs files

What does this MR do and why?

Some minor housekeeping: While working on #424990 (closed), I was gathering data around the present state of db/docs files, when I came across the fact that milestone can be empty for some files, which I got by running

Dir.glob("db/docs/*.yml").map do |file|
  YAML.load_file(file)
end.select { |data| data["milestone"] == nil }

on the console. This MR adds the right milestone to the files that are missing it.

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