fix: rollback engines to `>=16`

[styfle]

This is a partial revert of PR https://github.com/vercel/nft/pull/374

It was dropping support for node@16 in CI but it didn't modify target in tsconfig.json, so 16 is still supported.

Furthermore, 16 is going to be supported by Vercel a little longer.

This fixes the warning when installing Vercel CLI using node@16

npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: '@vercel/nft@0.26.1',
npm WARN EBADENGINE   required: { node: '>=18' },
npm WARN EBADENGINE   current: { node: 'v16.18.1', npm: '8.19.2' }
npm WARN EBADENGINE }

[github-actions[bot]]

:tada: This PR is included in version 0.26.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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