Update to modern style cast

[bmagistro]

When compiling with clang 19.1.7 on Alma with -Werror and a number of other flags (we run with most warnings enabled), the line is flagged with error: use of old-style cast [-Werror, -Wold-style-cast]. It is unclear if this was just missed or cast style was not updated.

Env: Alma 9.6 Boost 1.88.0 via cmake clang 19.1.7