improve the deprecation warning for with_optimizer

[mlubin]

I saw this warning and found it confusing:

julia> solver = with_optimizer(ECOS.Optimizer, verbose=0)
┌ Warning: `with_optimizer` is deprecated, replace `with_optimizer(Ipopt.Optimizer, max_cpu_time=60.0)`
│ by `optimizer_with_attributes(Ipopt.Optimizer, "max_cpu_time" => 60.0)`.

I don't have the line with_optimizer(Ipopt.Optimizer, max_cpu_time=60.0) in my code, so why is the message telling me to replace it?

IMO this should trigger 0.21.1, given that it affects all users updating from 0.20.

[codecov]

Codecov Report

Merging #2165 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2165   +/-   ##
=======================================
  Coverage   91.12%   91.12%           
=======================================
  Files          42       42           
  Lines        4112     4112           
=======================================
  Hits         3747     3747           
  Misses        365      365

Impacted Files	Coverage Δ
src/JuMP.jl	73% <ø> (ø)	⬆️

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