chore: migrate invite users to rtk

[tiagoapolo]

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Changes

migrates /invite api calls to RTK

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Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-5196	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-5196	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-5196	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5196	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5196	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-5196	Finished ✅	Results ✅

[kyle-ssg]

These migrations should involve removing the old stuff? I think there's quite a lot that'd be no longer in use