UPDATING.md entry about dashboard v2 forced migration in 0.28

[mistercrunch]

Getting ready to ship 0.28.0 ...

[codecov-io]

Codecov Report

Merging #5750 into master will increase coverage by 12.75%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #5750       +/-   ##
===========================================
+ Coverage   63.43%   76.19%   +12.75%     
===========================================
  Files         362       46      -316     
  Lines       22983     9257    -13726     
  Branches     2557        0     -2557     
===========================================
- Hits        14580     7053     -7527     
+ Misses       8388     2204     -6184     
+ Partials       15        0       -15

Impacted Files	Coverage Δ
superset/data/countries.py	0% <0%> (-100%)	⬇️
superset/viz.py	75.25% <0%> (-6.11%)	⬇️
superset/db_engine_specs.py	51.86% <0%> (-2.3%)	⬇️
superset/utils.py	87.88% <0%> (-1.27%)	⬇️
superset/models/core.py	84.76% <0%> (-0.34%)	⬇️
superset/views/core.py	73.71% <0%> (-0.23%)	⬇️
...sets/src/explore/components/ExploreChartHeader.jsx
superset/assets/src/components/Checkbox.jsx
...perset/assets/src/dashboard/util/getEmptyLayout.js
...ts/src/dashboard/util/getComponentWidthFromDrop.js
... and 312 more

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[betodealmeida]

+1