deprecations.jl: depwarn -> Base.depwarn

[skleinbo]

Because depwarn is not imported from Base, calling update!(...) throws an UndefVarError.
Changed depwarn to Base.depwarn.

See https://discourse.julialang.org/t/flux-13-3-error-undefvarerror-depwarn-not-defined/82118

PR Checklist

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable

No tests for depwarns exist as far as I can tell, so I did not add one.
Changes to NEWS.md and the docs aren't necessary in my opinion.

[CarloLucibello]

@skleinbo thank you for this. Since you are at it, would you mind also changing the second code block in this docs page to

η = 0.1 # Learning Rate
for p in (W, b)
  p .-= η * grads[p]
end

[skleinbo]

@CarloLucibello Sure!