Resolve "EXTERN: Adapt to Conan 2.0"

Summary

Related to #73 (closed) and #80 (closed): Conan 2.0 required several changes, both on the conanfile.py as well as the CMakeLists.txt file(s). Fixing #82 needs to be done before completing the merge.

Focus

The biggest changes are in conanfile.py and CMakeLists.txt all according to the migration guide. Crucial here are

• Conan now doesn't appear in the CMakeLists.txt files anymore. Instead, Conan generate CMakeToolChain files which handles all the necessary includes etc.
• These are included in our CMakeLists.txt using CMake's FindPackage

Test Environment

How to set up a test environment for manual testing?

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.