fix: flagsmith stale flags check

[kyle-ssg]

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• I have added information to docs/ if required so people know about the feature!
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• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

Fix flagsmith flag check, it was incorrectly using the isomorphic library instead of the standard, this also uses our standard util function.

How did you test this code?

Validated the site loads correctly and the flag is evaluated

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 12, 2024 3:05pm
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 12, 2024 3:05pm

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)			Nov 12, 2024 3:05pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4831	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4831	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-4831	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4831	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4831	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-4831	Finished ✅	Results ✅