fix(config): preserve min-release-age after flattening

[lawrence3699]

Fixes #9199.

min-release-age derives a before date while config is flattened, but the flatten hook also deleted the original min-release-age value from the loaded config data. After anything read config.flat, npm config get min-release-age and npm config ls --long reported null even when the user had configured a value.

This keeps the configured min-release-age value readable while still deriving flat.before for package resolution.

Before this change:

node . config get min-release-age --userconfig "$tmp/npmrc"
# null

After this change:

node . config get min-release-age --userconfig "$tmp/npmrc"
# 5

Validation:

npm test --ignore-scripts
npm run lint --ignore-scripts --workspace @npmcli/config
npm run postlint --ignore-scripts --workspace @npmcli/config
git diff --check