[ci] Only notify discord on core team label

[poteto]

Rather than notify on every label event on a PR by a core team member, only do so for the specific core team label event.

[react-sizebot]

Comparing: 10a4c88...1d1b5db

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	514.52 kB	514.52 kB	=	91.76 kB	91.76 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	557.55 kB	557.55 kB	=	99.01 kB	99.01 kB
facebook-www/ReactDOM-prod.classic.js	=	596.50 kB	596.50 kB	=	104.89 kB	104.89 kB
facebook-www/ReactDOM-prod.modern.js	=	586.92 kB	586.92 kB	=	103.35 kB	103.35 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 66d87ee

[rickhanlonii]

What about instead of using the label, we used the PR created event?

on:
  pull_request_target:
    types: [opened]
    paths-ignore:

the only reason I used the label before is that we didn't have the core team info in the repo