Omit isomorphic-fetch and isomorphic-form-data

[mmktomato]

Fetch API of Node.js was marked as stable on v21.
https://nodejs.org/en/blog/announcements/v21-release-announce#stable-fetchwebstreams

We can omit isomorphic-fetch and isomorphic-form-data after Node.js v20 EOL.
According to their release plan, it will be on 2026 July.
https://nodejs.org/en/about/previous-releases

Affected files

• package.json
  • https://github.com/nulab/backlog-js/blob/master/package.json#L31
  • https://github.com/nulab/backlog-js/blob/master/package.json#L42-L43
• README.md
  • https://github.com/nulab/backlog-js/blob/master/README.md?plain=1#L17-L18
• Others? Need to grep.

[mmktomato]

This resolves #2