Compilation error on Apple Clang 12.0 due to `std::bind_front` usage in version 3.42

Version 3.42 uses the std::bind_front function here, which is a C++20 feature. However, Apple Clang 12.0 does not support this function, causing the following compilation error:

/Users/clubie/Documents/Projects/test/ns-3-allinone/ns-3.42/src/core/model/timer-impl.h:155:29: error: no member named 'bind_front' in namespace 'std'
            : m_memPtr(std::bind_front(memPtr, objPtr))
                       ~~~~~^

To solve this issue, we can either:

Replace std::bind_front to a lambda expression, or provide our own implementation of std::bind_front, or
Increase the minimum required version of Apple Clang in CMakeLists.txt.

Steps to reproduce the issue:

# Prepare Apple Clang 12.0
>>> `which cc` --version
Apple clang version 12.0.5 (clang-1205.0.22.9)
Target: arm64-apple-darwin23.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

# Download and build ns-3 v3.42
>>> git clone https://gitlab.com/nsnam/ns-3-allinone.git
>>> cd ns-3-allinone
>>> python ./download.py -n ns-3.42
>>> ./build.py --enable-examples --enable-tests

I'm willing to create a merge request to address this problem if needed.