Add Halium 15 branch

[Umeaboy]

I fail to see why there are Halium branches up to 14, but not 15.
I therefor ask you to add it so that people can add devices that runs Lineage 22.1 before porting Halium or Ubports or similar projects.
My device (Samsung Galaxy S21 5G) can run Lineage 22.1 using the exynos-republic repos, but failing to build against the Halium branch for Android 15 (which Lineage 22.1 is based on) is not to an advantage.