Replaced void cast with maybe_unused

[Sanjeeb-Poudel]

Replaced void cast with the more modern [[maybe_unused]].

Working towards fixing #17247

[tamiko]

/rebuild

[tamiko]

@Sanjeeb-Poudel You'll need to update your local git configuration:

Invalid author 'sanjeeb' without firstname and lastname

For possible solutions, consult the following page:
      https://github.com/dealii/dealii/wiki/Commit-authorship