Update exprtk.hpp to release from 2024

What this MR does / why does DuMux need it:

Update exprtk.hpp to latest release.

There is good news and sad news.
Good:

• The new release fixes warnings for upcoming compiler releases.

Sad:

• The author pulled back from GitHub.
• Recent pull requests got closed.
• There is no version.
• No public Git repository, no more Git tag.

I am unsure what this means for the announced new release based on C++ 17.

Notes for the reviewer

Software Heritage has no entry for this. Did you create them the last time?