ci: Use beta-plugin-slug in autorelease action (#37863)

If no `.extra.autorelease.slug` and no `.extra.wp-plugin-slug` exist,
but `.extra.beta-plugin-slug` does, use that.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/9505993312

Upstream-Ref: Automattic/jetpack@4cfe43b

.github/files/autorelease.sh

@@ -28,7 +28,7 @@ if [[ ! -f composer.json ]]; then
 	exit 1
 fi
 
-SLUG="$(jq -r '.extra.autorelease.slug? // .extra["wp-plugin-slug"] // ( .name | sub( "^.*/"; "" ) )' composer.json)"
+SLUG="$(jq -r '.extra.autorelease.slug? // .extra["wp-plugin-slug"] // .extra["beta-plugin-slug"] // ( .name | sub( "^.*/"; "" ) )' composer.json)"
 if [[ -z "$SLUG" ]]; then
 	echo '::error::Failed to get slug from composer.json.'
 	exit 1