Remove python 3.8 nightly builds (pytorch#132138)

Removing python 3.8 support in nightly builds. As per PR: pytorch#120718
Pull Request resolved: pytorch#132138
Approved by: https://github.com/albanD, https://github.com/malfet, https://github.com/huydhn

Author: atalman

.github/scripts/generate_binary_build_matrix.py

@@ -215,7 +215,7 @@ def arch_type(arch_version: str) -> str:
     ("cpu", CXX11_ABI): f"pytorch/libtorch-cxx11-builder:cpu-{DEFAULT_TAG}",
 }
 
-FULL_PYTHON_VERSIONS = ["3.8", "3.9", "3.10", "3.11", "3.12"]
+FULL_PYTHON_VERSIONS = ["3.9", "3.10", "3.11", "3.12"]
 
 
 def translate_desired_cuda(gpu_arch_type: str, gpu_arch_version: str) -> str: