🐛 Route GitHub rate-limit errors to rate-limit page

[AnandChowdhary]

Summary

• Detect GitHub API rate-limit failures more robustly in the status-page client.
• Route secondary rate-limit messages and x-ratelimit-remaining: 0 errors to /rate-limit-exceeded instead of /error.
• Add focused Jest coverage for the error-routing helper.

Fixes upptime/upptime#1002

Test plan

• npm ci with Node 14.21.3 / npm 6.14.18
• RED: npm run unit-test -- --runTestsByPath src/utils/createOctokit.spec.js --runInBand failed before the fix
• GREEN: npm run unit-test -- --runTestsByPath src/utils/createOctokit.spec.js --runInBand
• npm run unit-test
• npm run build
• npm run test

Notes

• This repo's current master release job is already blocked by an invalid NPM_TOKEN from the previous merge, so I do not expect to merge/release this until that release-infra blocker is handled.