Bump u/xp to v1.15.3-up.1

[phisco]

Description of your changes

Part of https://github.com/upbound/universal-crossplane/issues/461

I have:

• [ ] Read and followed Upbound's contribution process.
• [ ] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested