Add deprecation warnings

[SvenKlaassen]

Description

Adding deprication warnings for dml_procedure and apply_cross_fitting.
Both arguments will be removed from the DoubleML in the future. Instead dml2 will be the standard method and crossfitting is automatically applied.
To still use non crossfitted learners external_predictions can be used in fit().

PR Checklist

• The title of the pull request summarizes the changes made.
• The PR contains a detailed description of all changes and additions.
• References to related issues or PRs are added.
• The code passes all (unit) tests.
• Enhancements or new feature are equipped with unit tests.
• The changes adhere to the PEP8 standards.