[CI] CI is broken for conan install tests, due to libcurl recipe changes.

[marcalff]

CI is now broken:

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Recipe 'libcurl/8.15.0' seems broken.
It is possible that this recipe is not Conan 2.0 ready
If the recipe comes from ConanCenter, report it at https://github.com/conan-io/conan-center-index/issues
If it is your recipe, check if it is updated to 2.0
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ERROR: Package 'libcurl/8.15.0' not resolved: libcurl/8.15.0: Cannot load recipe.
Error loading conanfile at '/home/runner/.conan2/p/libcu67a7edaf8a72c/e/conanfile.py': Current Conan version (2.15.1) does not satisfy the defined one (>=2.21.0)..
Error: Process completed with exit code 1.

Possible root cause:

conan-io/conan-center-index#28622

which changed this:

required_conan_version = ">=2.21.0"