Prefer C23 `memset_explicit()` over `explicit_bzero()`

[TimWolla]

Description

It appears we currently don't ever try to use memset_explicit(). We probably should feature test for it and do, since a standard function is likely to be safer / more reliable than a non-standard function.

[nielsdos]

See #15483

[TimWolla]

Should've checked the search, but also this should've already been done 🙈