[PR #140/eebe9958 backport][stable-2] Downgrade codecov uploader to v3 until it is no longer in a broken state

[patchback]

This is a backport of PR #140 as merged into main (eebe995).

It breaks with

Traceback (most recent call last):
  File "codecov_cli/main.py", line 81, in <module>
  File "codecov_cli/main.py", line 77, in run
  File "click/core.py", line 1157, in __call__
  File "click/core.py", line 1078, in main
  File "click/core.py", line 1688, in invoke
  File "click/core.py", line 1434, in invoke
  File "click/core.py", line 783, in invoke
  File "click/decorators.py", line 33, in new_func
  File "codecov_cli/commands/upload.py", line 243, in do_upload
  File "codecov_cli/services/upload/__init__.py", line 71, in do_upload_logic
  File "codecov_cli/services/upload/upload_collector.py", line 152, in generate_upload_data
  File "codecov_cli/services/upload/network_finder.py", line 17, in find_files
  File "codecov_cli/helpers/versioning_systems.py", line 111, in list_relevant_files
ValueError: Can't determine root folder

While the corresponding issue (codecov/codecov-action#1258) has been closed, it's not fixed, and there's only a workaround (codecov/feedback#263 (comment)) which, seriously, looks pretty dubious to me.