[refactor] check if c++23 standard is needed

[martin-raden]

Question: do we really need the c++23 support of the compiler or can we reduce to a lower standard to support older compilers as well?

• check if code is really requiring c++23 standard
• if not, adapt configure.ac to respective lower c++ standard

[Alexander-Mitrofanov]

Decision: retain C++23 as the current implementation baseline; no change to configure.ac.

The code uses std::string::resize_and_overwrite to construct ViennaRNA constraint strings directly in owned storage without redundant initialization, and std::string::contains for sequence checks. Changing only the configure requirement would not make the current source compatible with earlier standards.

This is an implementation and maintenance choice consistent with the GCC 14 / Apple Clang support policy. It is not a scientific requirement: stream views need only C++20, and the ownership and partition-map improvements use earlier standards. The recorded benchmarks preserve canonical outputs but do not demonstrate a material speedup attributable specifically to C++23.

A lower language baseline would require a separate compatibility change and validation.