fix(db): set slices.query_context to MEDIUMTEXT for mysql

[ktmud]

SUMMARY

Follow up for #20761, changing Slice.query_context and Slice.params to MEDIUMTEXT for MySQL databases. Ideally this should be ran before #20359 and #20346 but that would mean rebasing the existing migrations, which could be problematic for users who already ran both migrations.

We'll just accept the risk that some MySQL users may lose query_context data if they didn't manually run this column type change before #20359 and #20346...

On Airbnb's Superset instance with about 350k slices, this migration completes in about 50 seconds.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #20779 (f4fc2ba) into master (1d9d505) will decrease coverage by 11.48%.
The diff coverage is n/a.

❗ Current head f4fc2ba differs from pull request most recent head 8b2c592. Consider uploading reports for the commit 8b2c592 to get more accurate results

@@             Coverage Diff             @@
##           master   #20779       +/-   ##
===========================================
- Coverage   66.28%   54.79%   -11.49%     
===========================================
  Files        1756     1756               
  Lines       66737    66737               
  Branches     7049     7049               
===========================================
- Hits        44238    36571     -7667     
- Misses      20702    28369     +7667     
  Partials     1797     1797               

Flag	Coverage Δ
hive	53.27% <ø> (ø)
postgres	?
presto	53.13% <ø> (ø)
python	57.84% <ø> (-23.68%)	⬇️
sqlite	?
unit	50.28% <ø> (ø)

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

Impacted Files	Coverage Δ
superset/utils/dashboard_import_export.py	0.00% <0.00%> (-100.00%)	⬇️
superset/key_value/commands/update.py	0.00% <0.00%> (-88.89%)	⬇️
superset/key_value/commands/delete.py	0.00% <0.00%> (-85.30%)	⬇️
superset/key_value/commands/delete_expired.py	0.00% <0.00%> (-80.77%)	⬇️
superset/dashboards/commands/importers/v0.py	15.62% <0.00%> (-76.25%)	⬇️
superset/datasets/commands/update.py	25.30% <0.00%> (-68.68%)	⬇️
superset/datasets/commands/create.py	29.41% <0.00%> (-68.63%)	⬇️
superset/datasets/commands/importers/v0.py	24.03% <0.00%> (-67.45%)	⬇️
superset/reports/commands/execute.py	24.45% <0.00%> (-67.16%)	⬇️
superset/dashboards/dao.py	31.38% <0.00%> (-64.24%)	⬇️
... and 277 more