docs: 0.38.0 CHANGELOG and UPDATING

[dpgaspar]

SUMMARY

Updates the CHANGELOG and UPDATING for the new 0.38.0 release

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #11809 (9ece3d5) into master (f27ebc4) will decrease coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11809      +/-   ##
==========================================
- Coverage   67.45%   67.24%   -0.21%     
==========================================
  Files         915      915              
  Lines       44364    44364              
  Branches     4221     4221              
==========================================
- Hits        29925    29833      -92     
- Misses      14336    14428      +92     
  Partials      103      103              

Flag	Coverage Δ
cypress	55.12% <ø> (-0.10%)	⬇️
javascript	62.95% <ø> (ø)
python	63.57% <ø> (-0.32%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/db_engine_specs/mysql.py	79.59% <0.00%> (-12.25%)	⬇️
superset/db_engine_specs/sqlite.py	65.62% <0.00%> (-9.38%)	⬇️
...et-frontend/src/messageToasts/components/Toast.tsx	91.66% <0.00%> (-8.34%)	⬇️
superset/databases/commands/create.py	83.67% <0.00%> (-8.17%)	⬇️
superset/databases/commands/update.py	85.71% <0.00%> (-8.17%)	⬇️
...perset-frontend/src/messageToasts/actions/index.ts	88.46% <0.00%> (-7.70%)	⬇️
superset/utils/celery.py	96.42% <0.00%> (-3.58%)	⬇️
superset/models/core.py	85.86% <0.00%> (-2.99%)	⬇️
superset/datasets/api.py	88.72% <0.00%> (-2.95%)	⬇️
superset/views/core.py	71.59% <0.00%> (-2.63%)	⬇️
... and 8 more

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