Adjust beats-packaging-pipeline to trigger on 8.x

[rowlandgeoff]

I noticed that the beats-packaging-pipeline wasn't triggering automatically on the 8.x branch. Looks like adjusting this regex should fix that.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @rowlandgeoff? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.