Add relnotes-needs-review label

[clarfonthey]

See Zulip thread: #t-release > `relnotes-needs-review` tag?

Already added the label to the rust-lang/rust repo, this would just ensure it's also added to newly opened relnotes tracking issues.

[Urgau]

I don't really understand the point of adding yet another label, needs-triage works fine for this purpose.

[Urgau]

The change it-self looks fine. So if T-release wants this, no strong objection merging it.

View changes since this review

[clarfonthey]

Main issue is that needs-triage has an explicit purpose and this isn't really that purpose, so many people just remove it to avoid noise and it doesn't really help track what needs to be done here.

[Urgau]

Then I must be in the small minority that uses it for that purpose. ;-)