Rename UMaskExt to UMask2 for Core events

The goal of this issue is to document renaming the core event attribute `UMaskExt` to `UMask2`. The goal is to align with Unit Mask 2 as used in the Intel Software Developer's Manual. This change only applies to **core** event files.

## Example event https://github.com/intel/perfmon/blob/e49581aeb2903dde6fb1d187e9d412df58e01038/PTL/events/pantherlake_cougarcove_core.json#L469-L495

In the above example `UMaskExt` will be renamed to `UMask2` to align with the SDM. Intel SDM reference snippets.

---

---

## Reference Links

1. https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html
2. [Intel® 64 and IA-32 Architectures Software Developer's Manual Combined Volumes: 1, 2A, 2B, 2C, 2D, 3A, 3B, 3C, 3D, and 4](https://cdrdv2.intel.com/v1/dl/getContent/671200)