Adding a note about 0.30 to updating

[michellethomas]

Adding a note in UPDATING.md about the recommended downtime for 0.30.

@mistercrunch @xtinec @graceguo-supercat

[xtinec]

Thanks for noting this! 🙏

[codecov-io]

Codecov Report

Merging #6730 into master will increase coverage by 12.93%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #6730       +/-   ##
===========================================
+ Coverage    42.8%   55.73%   +12.93%     
===========================================
  Files         446      521       +75     
  Lines       13105    23586    +10481     
  Branches     2772     2935      +163     
===========================================
+ Hits         5609    13146     +7537     
- Misses       7086     9937     +2851     
- Partials      410      503       +93

Impacted Files	Coverage Δ
superset/assets/src/query/index.ts	100% <0%> (ø)	⬆️
superset/assets/src/query/buildQueryObject.ts	100% <0%> (ø)	⬆️
superset/assets/src/components/Hotkeys.jsx	100% <0%> (ø)	⬆️
superset/assets/src/query/Metric.ts	100% <0%> (ø)	⬆️
superset/assets/src/visualizations/nvd3/NVD3Vis.js	0% <0%> (ø)	⬆️
superset/assets/src/query/FormData.ts	100% <0%> (ø)	⬆️
...ts/src/explore/components/controls/FilterPanel.tsx	66.66% <0%> (ø)	⬆️
.../assets/src/visualizations/wordcloud/buildQuery.ts	100% <0%> (ø)	⬆️
superset/assets/src/query/buildQueryContext.ts	100% <0%> (ø)	⬆️
superset/assets/src/query/Column.ts	100% <0%> (ø)	⬆️
... and 79 more

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