1027 Use FetchContent_MakeAvailable instead of FetchContent_Populate

[reneSchm]

Changes and Information

Please briefly list the changes (main added features, changed items, or corrected bugs) made:

• Replace the deprecated FetchContent_Populate.
• Increase CMake minimum version to 3.18.
• Remove unused or outdated CMake files and code.

If need be, add additional information and what the reviewer should look out for in particular:

• Automatic fallbacks from FetchContent to find_package (or vice versa) via OVERRIDE_FIND_PACKAGE or FIND_PACKAGE_ARGS are only supported since cmake 3.24, which is newer than the most recent version in Ubuntu 22 (3.22)
• We may want to keep 1027 open if we want to reconsider automatic fallbacks in the future. Although I find the current solution is fine.

Merge Request - Guideline Checklist

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Checks by code author

• Every addressed issue is linked (use the "Closes #ISSUE" keyword below).
• New code adheres to coding guidelines.
• No large data files have been added (files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.).
• Tests are added for new functionality and a local test run was successful (with and without OpenMP).
• Appropriate documentation within the code (Doxygen) for new functionality has been added in the code.
• Appropriate external documentation (ReadTheDocs) for new functionality has been added to the online documentation and checked in the preview.
• Proper attention to licenses, especially no new third-party software with conflicting license has been added.
• (For ABM development) Checked benchmark results and ran and posted a local test above from before and after development to ensure performance is monitored.

Checks by code reviewer(s)

• Corresponding issue(s) is/are linked and addressed.
• Code is clean of development artifacts (no deactivated or commented code lines, no debugging printouts, etc.).
• Appropriate unit tests have been added, CI passes, code coverage and performance is acceptable (did not decrease).
• No large data files added in the whole history of commits(files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.).
• On merge, add 2-5 lines with the changes (main added features, changed items, or corrected bugs) to the merge-commit-message. This can be taken from the briefly-list-the-changes above (best case) or the separate commit messages (worst case).

Closes #1027

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.37%. Comparing base (20cee90) to head (3f062e5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1535      +/-   ##
==========================================
- Coverage   97.38%   97.37%   -0.02%     
==========================================
  Files         188      188              
  Lines       16619    16619              
==========================================
- Hits        16184    16182       -2     
- Misses        435      437       +2     

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