[CI][ArmPL] Upgrade DPC++ version in Arm CI and use apt to install ArmPL

[rafbiels]

Two changes improving the CI configuration running on Arm CPUs:

1. Use apt to install ArmPL in the CI

Install ArmPL from apt (as per instructions) instead of downloading and installing binaries. This has the benefit of always testing against the latest version instead of having to manually update the hard-coded one.

2. Upgrade DPC++ version in Arm CI pipeline to 6.1.0

A nightly tag was previously used as no release version working on the GitHub Arm runners had been available. Version 6.1.0 was released recently which supports the architecture and software stack in our CI pipeline.