Update current nightly version to 2.8.0 (#6442)

Should fix docker build verification issues, like in:
https://github.com/pytorch/pytorch/actions/runs/13917778953/job/38943972867

- In the PyTorch repo, version.txt shows 2.8.0a0 (i.e., PyTorch is now
developing version 2.8.0)
- In the test-infra repo's generate_binary_build_matrix.py, line 77
shows CURRENT_NIGHTLY_VERSION = "2.7.0"
- Build job builds:
ghcr.io/pytorch/pytorch-nightly:2.8.0.dev20250318-cuda11.8-cudnn9-devel
- Validation job tries to pull:
ghcr.io/pytorch/pytorch-nightly:2.7.0.dev20250318-cuda11.8-cudnn9-devel
- The image doesn't exist, causing "manifest unknown" errors

Author: izaitsevfb

tools/scripts/generate_binary_build_matrix.py

@@ -74,7 +74,7 @@
 XPU = "xpu"
 
 
-CURRENT_NIGHTLY_VERSION = "2.7.0"
+CURRENT_NIGHTLY_VERSION = "2.8.0"
 CURRENT_CANDIDATE_VERSION = "2.7.0"
 CURRENT_STABLE_VERSION = "2.6.0"
 CURRENT_VERSION = CURRENT_STABLE_VERSION