Provide deprecation path for AIIDA_CONFIG_FOLDER

[khsrali]

Regarding merged PR #6610, I believe we still need to support AIIDA_CONFIG_FOLDER within a deprecation path.

aiida-workgraph and aiida-pythonjob now fail, because they used to use the public method AIIDA_CONFIG_FOLDER.

[unkcpz]

The AIIDA_CONFIG_FOLDER was never recommended as public APIs. IMO, the deprecation warnings should only put for public APIs.
Instead of provide deprecation path, if it is necessary to expose it as public API, we should do that first.

[unkcpz]

There are exposed API for getting the config path

from aiida.manage import get_config
config = get _config()
config_path = config.dirpath