Fix: metall compat with boost >= 1.85

[liss-h]

Fix compat with recent boost releases.

Issue

Previously construct_n had an additional out parameter that told you how many elements were successfully constructed, this is no longer the case.

Compare the impl in this with the original boost impl:
https://github.com/boostorg/interprocess/blob/a0c5a8ff176434c9024d4540ce092a2eebb8c5c3/include/boost/interprocess/detail/segment_manager_helper.hpp#L211-L224

[liss-h]

Since the issue explained in the PR description is present, it does not feel safe to actually include this.

I will close this PR, we'll just have to use an older boost version.