Update Vercel adapter to Node 14.8 to fix top level await builds

[digitaldeus]

Describe the bug

SvelteKit has been updated to node14.8 to enable top-level await per #4742 . Would be great if we could update the vercel adapter to the same version since it is currently forcing build version node14 and there doesn't appear to be a way to override it

Reproduction

repo here

to repro just run:

npm install
npm run build

Logs

> Using @sveltejs/adapter-vercel
✘ [ERROR] Top-level await is not available in the configured target environment ("node14")

    .svelte-kit/output/server/chunks/hooks-45a47dbd.js:1:0:
      1 │ await Promise.resolve(true);
        ╵ ~~~~~

> Build failed with 1 error:
.svelte-kit/output/server/chunks/hooks-45a47dbd.js:1:0: ERROR: Top-level await is not available in the configured target environment ("node14")
.svelte-kit/output/server/chunks/hooks-45a47dbd.js:1:0: ERROR: Top-level await is not available in the configured target environment ("node14")
    at failureErrorWithLog (/Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:1603:15)
    at /Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:1249:28
    at runOnEndCallbacks (/Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:1162:65)
    at buildResponseToResult (/Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:1247:7)
    at /Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:1356:14
    at /Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:666:9
    at handleIncomingPacket (/Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:763:9)
    at Socket.readFromStdout (/Users/deusduke/src/vercel-top-level-await/node_modules/esbuild/lib/main.js:632:7)
    at Socket.emit (node:events:527:28)
    at addChunk (node:internal/streams/readable:324:12)

System Info

System:
    OS: macOS 12.3.1
    CPU: (8) arm64 Apple M1
    Memory: 96.47 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 17.8.0 - /opt/homebrew/bin/node
    Yarn: 1.22.18 - ~/.yarn/bin/yarn
    npm: 8.5.5 - /opt/homebrew/bin/npm
  Browsers:
    Brave Browser: 100.1.37.116
    Chrome: 100.0.4896.127
    Safari: 15.4

Severity

blocking all usage of SvelteKit

Additional Information

Unable to use sveltekit in vercel due to top level await issue

[antony]

I might suggest that we instead read the node version from the application's config, rather than just update the hardcoded version inside the adapter.

[digitaldeus]

Vercel now support Node16 so allowing the adapter to be dynamic would be a great approach