[CMAKE] Don't set iwyu and clang-tidy properties on nlohmann_json with cmake <3.19

[dbarker]

Fixes #3560

nlohmann_json is an INTERFACE target and CMake <3.19 does not support setting the iwyu and clang-tidy properties on interface targets.

Changes

• check if the CMake version is >= 3.19 before setting the iwyu and clang-tidy properties on the nlohmann-json target
• Update the fetch content test in the CI workflow to verify fetching nlohmann-json (and the other fetchable dependencies) with the minimum supported CMake version (3.14).

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.01%. Comparing base (dbd0680) to head (26003c4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3568      +/-   ##
==========================================
- Coverage   90.03%   90.01%   -0.01%     
==========================================
  Files         220      220              
  Lines        7065     7065              
==========================================
- Hits         6360     6359       -1     
- Misses        705      706       +1     

see 1 file with indirect coverage changes

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[marcalff]

LGTM, thanks for the fix.

[marcalff]

Note that we may have to upgrade CMake in the future, because

https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

is now at CMake >= 3.22 since 2025-06-04.

Still, it is good to have this fix in the code base.