Upgrade y18n package to v4.0.1

[kyoto]

Upgrade y18n package (used by the React UI) from v4.0.0 to v4.0.1, which fixes CVE-2020-7774.

See https://github.com/yargs/y18n/blob/v4/CHANGELOG.md#401-2020-11-30

(Issue is also fixed in y18n v5.0.5)

This is the same issue as prometheus/prometheus#8282

[kakkoyun]

@kyoto Thanks for opening this. We should take care of it. Help wanted.
Also, it's curious that we haven't alerted by the dependabot?

[stale]

Hello 👋 Looks like there was no activity on this issue for the last two months.
Do you mind updating us on the status? Is this still reproducible or needed? If yes, just comment on this PR or push a commit. Thanks! 🤗
If there will be no activity in the next two weeks, this issue will be closed (we can always reopen an issue if we need!). Alternatively, use remind command if you wish to be reminded at some point in future.

[kyoto]

This was fixed by #3813