Update MIGRATION.md

[Hypnosphi]

There are some breaking changes in 3.3 that arent reflected in migration guide

[Hypnosphi]

TODO: replace "we're revisiting" with "we've revisited" once we update the RELEASES.md file

[codecov]

Codecov Report

Merging #2709 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2709   +/-   ##
=======================================
  Coverage   34.35%   34.35%           
=======================================
  Files         390      390           
  Lines        8772     8772           
  Branches      898      896    -2     
=======================================
  Hits         3014     3014           
+ Misses       5166     5155   -11     
- Partials      592      603   +11

Impacted Files	Coverage Δ
app/vue/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/react/src/server/utils.js	0% <0%> (-53.58%)	⬇️
...tive/src/preview/components/StoryListView/index.js	0% <0%> (ø)	⬆️
lib/ui/src/libs/withLifecycleDecorator.js	17.14% <0%> (ø)	⬆️
lib/ui/src/libs/key_events.js	33.96% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Rules.js	0% <0%> (ø)	⬆️
addons/info/src/components/Story.js	88.88% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/shortcuts_help.js	25% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/routed_link.js	22.72% <0%> (ø)	⬆️
lib/ui/src/modules/ui/index.js	0% <0%> (ø)	⬆️
... and 40 more

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[shilman]

LGTM!