[MPFR] Temporarily set version back to 4.1.1 for rebuild

[ararslan]

There were no functional changes between #5904 and now; all that should be needed is the version change. Once merged, I'll revert this with [skip build].

[giordano]

I'll revert this with [skip build].

And [skip ci] upon merging (but not in the PR, otherwise CI will never run and the PR can't be merged)

[ararslan]

Checking my understanding: I put [skip build] in the commit message for the commit I push to a branch, and I include [skip ci] in the commit message that GitHub allows you to edit when merging the PR?

[giordano]

Correct