Fix signif deprecation

[jmkuhn]

Update to PR #26670.

[jmkuhn]

With the current nightly:

julia> signif(1.23456, 3)
┌ Warning: `signif(x::Number, digits)` is deprecated, use `round(x; sigdigits=digits, base=base)` instead.
│   caller = top-level scope
└ @ Core :0
ERROR: MethodError: no method matching log(::typeof(base), ::Float64)
Closest candidates are:
  log(::Irrational{:ℯ}, ::Number) at mathconstants.jl:91
  log(::T<:Number, ::T<:Number) where T<:Number at math.jl:177
  log(::Number, ::Number) at math.jl:205
Stacktrace:
 [1] hidigit(::Float64, ::Function) at ./floatfuncs.jl:184
 [2] _round at ./floatfuncs.jl:190 [inlined]
 [3] #round#559 at ./floatfuncs.jl:128 [inlined]
 [4] #round at ./<missing>:0 [inlined] (repeats 2 times)
 [5] signif(::Float64, ::Int64) at ./deprecated.jl:57
 [6] top-level scope

instead of the desired:

julia> signif(1.23456, 3)
┌ Warning: `signif(x::Number, digits)` is deprecated, use `round(x; sigdigits=digits)` instead.
│   caller = top-level scope
└ @ Core :0
1.23