Rename `FPCategory`, `FP*` to `FpCategory`, `Fp*` to adhere to the naming convention

This is a [breaking-change].
Merge pull request #19758 from tbu-/pr_fp_name

Rename `FPCategory`, `FP*` to `FpCategory`, `Fp*` to adhere to the naming convention

Reviewed-by: Gankro