For #6017

• Delete everything under src/client/debugger/debugAdapter : see Remove TS debugAdapter #11755
• Delete installOldPtvsd gulp task and refactor the installPythonRequirements gulp task (it won't be a series of tasks anymore)
• Delete CI steps & gulp task installing ptvsd
• Remove packaging, pyparsing and six from requirements.txt
• Remove DebugAdapterNewPtvsd and DebugAdapterDescriptorFactory experiments
  - [ ] Remove pythonFiles/debug_adapter and clean up pythonFiles/tests/__main__.py
  Edit: The tests under pythonFiles/debug_adapter test the wheels installer for debugpy. We should keep it.
• Remove packaging from test-requirements.txt
• Remove legacy code in buildApi() : see Remove APIs related to ptvsd paths #11757
  - [ ] Remove pin of the Node version in the GitHub Action CI workflow (Pin the version of Node used in GHA  #10410 (comment))
  - [ ] Remove pin of the Node version in the AzDO workflows (NodeVersion in globals.yml and all the places using it)
  Edit: We have decided to pin node version for consistency across all platforms.
• Remove all uses of ChildProcessLaunchData, isChildProcessLaunchData, and ChildProcessLaunched