[lldb] Recommend Python 3.8 as the minimum Python version for LLDB (#114807)
See
https://discourse.llvm.org/t/rfc-lets-document-and-enforce-a-minimum-python-version-for-lldb/82731
for discussions.
This matches LLVM's requirement to run tests. For LLDB 20 there will be
a CMake warning telling builders that from LLDB 21 this will be a hard
requirement. From LLDB 21, it will be an error to try to build with
anything <= 3.8.
So there are no code changes in this commit. Once the llvm 20 branch is
created we can remove some < 3.8 support code.
As always, if you disable Python support you will not get any new
warnings or errors from this change.