fix(nextjs): cleanup old options for server executor (proxyConfig)

[ndcunningham]

The implementation for proxyConfig was already removed since we are now using Next.js' CLI.

This is just cleaning up and removing the deprecation message,

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
nx-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 16, 2023 2:54pm

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✅ Successfully ran 8 targets

• nx affected -t e2e-macos --parallel=1 --base=bb88dd934a07bf5a5291abbe82d95ddcd68d3c84 --head=245683ff9d9cbae28de769ca65b64f8ccfb8e253
• nx affected --target=e2e --base=bb88dd934a07bf5a5291abbe82d95ddcd68d3c84 --head=245683ff9d9cbae28de769ca65b64f8ccfb8e253 --parallel=1
• nx affected --target=build --base=bb88dd934a07bf5a5291abbe82d95ddcd68d3c84 --head=245683ff9d9cbae28de769ca65b64f8ccfb8e253 --parallel=3
• nx affected --target=test --base=bb88dd934a07bf5a5291abbe82d95ddcd68d3c84 --head=245683ff9d9cbae28de769ca65b64f8ccfb8e253 --parallel=1
• nx run-many -t check-imports check-commit check-lock-files depcheck check-codeowners documentation --parallel=1 --no-dte
• nx affected --target=lint --base=bb88dd934a07bf5a5291abbe82d95ddcd68d3c84 --head=245683ff9d9cbae28de769ca65b64f8ccfb8e253 --parallel=3
• nx-cloud record -- nx format:check --base=bb88dd934a07bf5a5291abbe82d95ddcd68d3c84 --head=245683ff9d9cbae28de769ca65b64f8ccfb8e253
• nx documentation --no-dte

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[jaysoo]

LGTM!

[github-actions]

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