Request failed with status code 400

## Description

$ gridsome develop
Gridsome v0.7.23

Initializing plugins...
Error: Request failed with status code 400
at createError (/Users/aily/Desktop/gridsome/my-gridsome-site/node_modules/contentful/dist/contentful.node.js:886:15)
at settle (/Users/aily/Desktop/gridsome/my-gridsome-site/node_modules/contentful/dist/contentful.node.js:1049:12)
at IncomingMessage.handleStreamEnd (/Users/aily/Desktop/gridsome/my-gridsome-site/node_modules/contentful/dist/contentful.node.js:294:11)
at IncomingMessage.emit (events.js:327:22)
at endReadableNT (_stream_readable.js:1221:12)
at processTicksAndRejections (internal/process/task_queues.js:84:21)

### Steps to reproduce

Everything works well when I connected to a new contentful account. But when I changed to the old contentful account and token. I got this error.

### Expected result

Expected to start without error.

### Actual result

$ gridsome develop
Gridsome v0.7.23

Initializing plugins...
Error: Request failed with status code 400
at createError (/Users/aily/Desktop/gridsome/my-gridsome-site/node_modules/contentful/dist/contentful.node.js:886:15)
at settle (/Users/aily/Desktop/gridsome/my-gridsome-site/node_modules/contentful/dist/contentful.node.js:1049:12)
at IncomingMessage.handleStreamEnd (/Users/aily/Desktop/gridsome/my-gridsome-site/node_modules/contentful/dist/contentful.node.js:294:11)
at IncomingMessage.emit (events.js:327:22)
at endReadableNT (_stream_readable.js:1221:12)
at processTicksAndRejections (internal/process/task_queues.js:84:21)

### Environment

System:
OS: macOS 10.15.7
CPU: (4) x64 Intel(R) Core(TM) i5-7360U CPU @ 2.30GHz
Binaries:
Node: 12.18.1 - ~/.nvm/versions/node/v12.18.1/bin/node
npm: 6.14.5 - ~/.nvm/versions/node/v12.18.1/bin/npm
Browsers:
Chrome: 86.0.4240.198
Safari: 14.0.1
npmPackages:
@gridsome/source-contentful: ^0.6.0 => 0.6.0
@gridsome/source-filesystem: ^0.6.2 => 0.6.2
@gridsome/transformer-remark: ^0.6.4 => 0.6.4
gridsome: ^0.7.0 => 0.7.23
npmGlobalPackages:
@gridsome/cli: 0.3.4

Start by reproducing `gridsome develop` with the older Contentful account and token, then inspect initialization in `@gridsome/source-contentful` and the reported Contentful request failure. Compare the old and new account configurations; done means the project starts successfully or the 400 response has a documented, actionable cause.

Written by the indexing model from the issue text.