Remove the old github conda workflow after the refactoring

[tramora]

---

TODO Before Asking for a Review

• Rebase your branch to the latest version of main (or main-v10)
• Make sure all CI workflows are green
• When adding a public feature/fix: Update the Unreleased section of CHANGELOG.md (no date)
• Self-Review: Review "Files Changed" tab and fix any problems you find
• API Docs (only if there are changes in docstrings, rst files or samples):
  • Check the docs build without warning: see the log of the API Docs workflow
  • Check that your changes render well in HTML: download the API Docs artifact and open index.html
  • If there are any problems it is faster to iterate by building locally the API Docs

[popescu-v]

Remove the line "- Update the default khiops-core version in [.github/workflows/conda.yml]
" in the CONTRIBUTING.md file, the "Public Releases" section.

I would also opportunistically update, in a separate commit, this section with an item on the usage of the new test-conda-forge-package.yml workflow, even though it is out of the scope of this PR.