update the JavaScript version policy to ES2021 (after June 2026)

[erwindon]

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Start using features from this JS version

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Start using useful features from this JS version.

The 12th edition, ECMAScript 2021, was published in June 2021. This version introduces the replaceAll method for strings; Promise.any, a promise combinator that short-circuits when an input value is fulfilled; AggregateError, a new error type to represent multiple errors at once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a target object without preserving it from garbage collection, and FinalizationRegistry, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (1_000); and Array.prototype.sort was made more precise, reducing the number of cases that result in an implementation-defined sort order.

Additional context
Update:

• docs/README.md (ecmaVersion)
• eslint.config.js
• any JS code