Upgraded github actions to v7 to resolve Node.js 20 deprecation

[tobyktbt]

Description

Upgrade GitHub Actions across all workflows to their latest versions to address the Node.js 20 deprecation warning:

• Upgrade actions/checkout from @v4 to @v7
• Upgrade actions/setup-python from @v5 to @v7
• Upgrade actions/cache from @v4 to @v6
• Upgrade actions/upload-artifact from @v4 to @v7

Updated workflow files:

• .github/workflows/deploy_docu_dev.yml
• .github/workflows/deploy_docu_stable.yml
• .github/workflows/test_build_docu_dev.yml
• .github/workflows/test_build_docu_released.yml

Reference to Issues or PRs

• Py counterpart: DoubleML/doubleml-for-py#388
• R counterpart: DoubleML/doubleml-for-r#228

Comments

Since this is my first PR, feedback or Improvement Suggestions about the processes or the pull request would be appreciated

PR Checklist

Please fill out this PR checklist (see our contributing guidelines for details).

• 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.