Do not deprecate classes Variable, Variables and Datasets on a specific version

[schlunma]

Remove the specific removal date of the classes Variable, Variables and Datasets since many diagnostics are using these.

• Closes Do not deprecate classes Variable, Variables and Datasets on a specific version #2280.

---

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🛠 This pull request has a descriptive title
• 🛠 Code is written according to the code quality guidelines
• 🛠 Documentation is available
• 🛠 Tests run successfully
• 🛠 The list of authors is up to date
• 🛠 Any changed dependencies have been added or removed correctly
• 🛠 All checks below this pull request were successful

---

To help with the number of pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository