Replace clipboard.js with async clipboard api (#15899)

Use async clipboard api [1] over this dependency, saving around 10kB
bundle size before minify while delivering the same functionality.

The issue comment button works but does not have a popup indication. We
could add some toast-style notifications in the future to fix that but I
think it's out of scope of this PR.

[1] https://developer.mozilla.org/en-US/docs/Web/API/Clipboard/writeText

package.json

@@ -9,7 +9,6 @@
     "@claviska/jquery-minicolors": "2.3.5",
     "@primer/octicons": "13.0.0",
     "add-asset-webpack-plugin": "2.0.1",
-    "clipboard": "2.0.8",
     "codemirror": "5.61.0",
     "css-loader": "5.2.4",
     "dropzone": "5.9.2",