Fix failing specs

Context

Some specs are failing on the default branch (see the main branch pipelines or a sample job)

Below are what we expect from the stages/groups structure in https://about.gitlab.com/stages.json:

expect(resource.first_group_for_stage('growth')).to eq('activation')

# [...]

expect(resource.stage_for_group('activation')).to eq('growth')

Here is what is currently found at this page:

$ curl --silent 'https://about.gitlab.com/stages.json' | jq .growth.groups
[
  "conversion",
  "expansion",
  "adoption",
  "product_intelligence"
]

# The first group is now `conversion`, and `adoption` has disappeared.

What does this MR do?

Fix the specs by using conversion as the first group of the growth stage.