update the JavaScript version policy to ES2026 (after June 2031)

**Is your feature request related to a problem? Please describe.**
Start using features from this JS version

**Describe the solution you'd like**
Start using useful features from this JS version.

> The 17th edition of ECMAScript released in 2026.
No summary available on Wikipedia yet.

**Additional context**
Update:
* docs/README.md (ecmaVersion)
* eslint.config.js
* any JS code