[CMAKE] Upgrade curl to 8.21.0

[dbarker]

Contributes to #4163

Changes

• upgrade curl to 8.21.0

• curl 8.21.0 has a cmake minimum of 3.18 and the otel-cpp minimum is 3.16. This only impacts the FetchContent (and add_subdirectory) use case. Many other dependencies are still at 3.16 (grpc, protobuf, googletest) so bumping the project CMake minimum is not appropriate right now. Instead this PR:

  • Updates the FetchContent test is updated to install CMake 3.18
  • Updates the INSTALL.md guide accordingly.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.89%. Comparing base (10a5a4f) to head (e491225).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4187   +/-   ##
=======================================
  Coverage   82.89%   82.89%           
=======================================
  Files         405      405           
  Lines       17292    17292           
=======================================
  Hits        14332    14332           
  Misses       2960     2960           

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