Improving compatibility with fmt 10

[dschwoerer]

Todo:

• check this resolves the FTBFS

[ZedThree]

Fedora is failing with:

Unknown argument "builddep" for command "dnf5". Add "--help" for more information about the arguments.

But there's also a compile error:

/home/runner/work/BOUT-dev/BOUT-dev/src/solver/impls/imex-bdf2/imex-bdf2.cxx:1244:25: error: ‘SNESGetConvergedReasons’ was not declared in this scope; did you mean ‘SNESGetConvergedReason’?

The suggestion is wrong, and I think, in classic PETSc fashion, there's a completely different thing to use here: SNESGetConvergedReasonString

Also, we now use this pattern in quite a few places (I count at least 7), so might be worth wrapping up in a helper function?