Doctest is updated to v2.4.1

[YarikTH]

Doctest is updated to v2.4.1. Nothing more, nothing less

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• [-] The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header file single_include/nlohmann/json.hpp. The whole process is described here.

[coveralls]

Coverage remained the same at 100.0% when pulling 6390fca on YarikTH:feature/doctest_v2.4.1 into 260be04 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!