Update fmt version to 12.0.0

[servantftransperfect]

Upgrading fmt dependency to 12.0.0.

Current fmt version is not compatible with Clang 21.1.2 (Maybe older too)

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[lgritz]

I am not opposed to raising the auto-build version of fmt. There is another PR #4910 in review that updates several auto-build versions (which does not include fmt, but I think that is a mere oversight and not purposeful intent).

But this change is failing on Windows, so there's probably something else still needing adjustment for that platform.

Can you please explain more about how fmt 10.2 was failing against clang 21.1? Can you maybe reproduce some of the error log here so we can see?

Since auto-build is just used when a dependency is not found externally, if it's true that certain (moderately recent, even) versions of fmt don't work with the latest compilers, I feel like we should detect this case and warn about it with a clear explanation, even when not doing auto-build.