feat(nextjs): remove the "root" option from Next.js build executor since it can be inferred

[jaysoo]

We never needed the root option for @nx/next:build executor since we know the project root by reading the graph. This PR:

1. Removes the root option from said executor.
2. Update app generator to no longer generate with it.
3. Adds a migration to remove it from existing projects.

Current Behavior

Expected Behavior

Related Issue(s)

Fixes #

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✅ Successfully ran 7 targets

• nx affected -t e2e-macos --parallel=1 --base=39f745955b0d2f4fc6837075f977d1c48e658b35 --head=85cf4a431e328f0581bb5d3382d4d8dae566fe71
• nx affected --target=e2e --base=39f745955b0d2f4fc6837075f977d1c48e658b35 --head=85cf4a431e328f0581bb5d3382d4d8dae566fe71 --parallel=1
• nx affected --target=build --base=39f745955b0d2f4fc6837075f977d1c48e658b35 --head=85cf4a431e328f0581bb5d3382d4d8dae566fe71 --parallel=3
• nx affected --target=test --base=39f745955b0d2f4fc6837075f977d1c48e658b35 --head=85cf4a431e328f0581bb5d3382d4d8dae566fe71 --parallel=1
• nx run-many -t check-imports check-commit check-lock-files depcheck check-codeowners --parallel=1 --no-dte
• nx affected --target=lint --base=39f745955b0d2f4fc6837075f977d1c48e658b35 --head=85cf4a431e328f0581bb5d3382d4d8dae566fe71 --parallel=3
• nx-cloud record -- nx format:check --base=39f745955b0d2f4fc6837075f977d1c48e658b35 --head=85cf4a431e328f0581bb5d3382d4d8dae566fe71

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[github-actions]

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