Make PYTHONSTARTUP default to true

[anthonykim1]

Related: #24140

The default to enable shell integration in Python Terminal REPL via setting PYTHONSTARTUP environment variable has been set to false to target September release version of the Python extension.

• After stable release, we could roll the default as true for pre-release users ONLY in October, make sure we are not breaking anyone's Python terminal REPL.

• Eventually make default for PYTHONSTARTUP (enable shell integration for Python terminal REPL) as true for release/stable versions after some feedback in November.

[karthiknadig]

This should be in experiment. So, we should be able to switch it after the experiment is 100% both insiders and stable.