docs: clarify npm 12 unknown-config breaking change in changelogs (#9805)

## Summary

Fixes #9802.

Before the npm 12 stable cut, #9729 restored warn-by-default for unknown
`.npmrc` keys (with `strict-npmrc` to opt into errors). #9733 updated
the `12.0.0-pre.1` changelog bullet to match, but the aggregated stable
`12.0.0` / `@npmcli/config@11.0.0` breaking-change notes (and the config
package's `pre.1` note) still said unknown `.npmrc` configs throw.

That stale line also shipped in the published [v12.0.0 GitHub release
notes](https://github.com/npm/cli/releases/tag/v12.0.0).

This aligns those changelog bullets with the corrected wording:

> unknown CLI flags, abbreviated flags, and single-hyphen multi-char
shorthands now throw instead of warning. (Unknown `.npmrc` configs still
warn by default; opt into erroring with the new `strict-npmrc` config.)

Maintainers may also want to refresh the published `v12.0.0` release
body to match; that cannot be updated via this PR alone.

## References

- #9729 (warn instead of error on unknown `.npmrc` configs)
- #9733 (clarified the `pre.1` changelog note)