Bump webpack

[hughhhh]

let's upgrade our webpack package. Things look good on my end developing with the new version. I'm not sure if we had any reason not to update.

@williaster @kristw @mistercrunch

[codecov-io]

Codecov Report

Merging #5744 into master will decrease coverage by 0.64%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #5744      +/-   ##
=========================================
- Coverage   63.45%   62.8%   -0.65%     
=========================================
  Files         362     363       +1     
  Lines       22978   23239     +261     
  Branches     2554    2622      +68     
=========================================
+ Hits        14580   14595      +15     
- Misses       8383    8629     +246     
  Partials       15      15

Impacted Files	Coverage Δ
superset/assets/src/explore/controls.jsx	43.25% <0%> (-3.02%)	⬇️
superset/assets/src/visualizations/sunburst.js	0% <0%> (ø)	⬆️
superset/assets/src/visualizations/horizon.js	0% <0%> (ø)

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[mistercrunch]

+1