fix(storybook): only check existence of storybook before migrating

[mandarini]

Change migrator to just check if Storybook is installed in the workspace.
Before, it would check if Storybook is already in v7. The thing here is that during the previous step of the migration, during packageJsonUpdates, Nx has already set Storybook to version 7, if Storybook exists in the workspace.
So, it makes no sense here to check if the version is 7, because it will always be. The migrate-7 generator will handle the logic of whether it needs to actually run any migrations or not.

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1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
nx-dev	⬜️ Ignored (Inspect)			Apr 20, 2023 11:09am

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✅ Successfully ran 8 targets

• nx affected -t e2e-macos --parallel=1 --exclude=e2e-detox
• nx affected --target=e2e --base=a51ac58a456607943469b286909afc4411e8b3a0 --head=85eb5d3d1bd7c3e595a4fe0ed4e07e5c90fd1f47 --parallel=1
• nx affected --target=test --base=a51ac58a456607943469b286909afc4411e8b3a0 --head=85eb5d3d1bd7c3e595a4fe0ed4e07e5c90fd1f47 --parallel=1
• nx affected --target=build --base=a51ac58a456607943469b286909afc4411e8b3a0 --head=85eb5d3d1bd7c3e595a4fe0ed4e07e5c90fd1f47 --parallel=3
• nx run-many -t check-imports check-commit check-lock-files depcheck --parallel=1 --no-dte
• nx affected --target=lint --base=a51ac58a456607943469b286909afc4411e8b3a0 --head=85eb5d3d1bd7c3e595a4fe0ed4e07e5c90fd1f47 --parallel=3
• nx-cloud record -- nx format:check --base=a51ac58a456607943469b286909afc4411e8b3a0 --head=85eb5d3d1bd7c3e595a4fe0ed4e07e5c90fd1f47
• nx documentation --no-dte

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