Fix deprecation of result_of in C++17

[softins]

Short description of changes

Fixes a deprecation warning for result_of used in threadpool.h, when compiling in C++17 or later. In this case, invoke_result_t should be used instead, with slightly different syntax. In the CI with this change, invoke_result_t is used for Android and macOS, and result_of is used on Linux, macOS Legacy, iOS and Windows.

CHANGELOG: Internals: fix deprecation warnings in threadpool.h for C++17 onwards

Context: Fixes an issue?

I discovered this deprecation while compiling on FreeBSD 15, and subsequently noticed it also occurs in the CI build for macOS (non-legacy). Thanks to ChatGPT for the suggested fix.

Does this change need documentation? What needs to be documented and how?

No. Compilation enhancement only.

Status of this Pull Request

Tested locally and on all CI platforms.

What is missing until this pull request can be merged?

Nothing.

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

AUTOBUILD: Please build all targets