chore: Add explicit ON DELETE CASCADE for embedded_dashboards

[john-bodley]

SUMMARY

Somewhat akin to #24628, this PR ensures explicit ON DELETE CASCADE operations are set for the embedded_dashboards table which means these relationships don't need to be explicitly deleted for non-SQLAlchemy ORM operations.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

CI.

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 #24939 (acda41f) into master (94c5950) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head acda41f differs from pull request most recent head a0e5621. Consider uploading reports for the commit a0e5621 to get more accurate results

@@            Coverage Diff             @@
##           master   #24939      +/-   ##
==========================================
- Coverage   69.00%   69.00%   -0.01%     
==========================================
  Files        1906     1906              
  Lines       74144    74141       -3     
  Branches     8211     8211              
==========================================
- Hits        51164    51161       -3     
  Misses      20856    20856              
  Partials     2124     2124              

Flag	Coverage Δ
hive	54.18% <100.00%> (+<0.01%)	⬆️
mysql	79.21% <100.00%> (-0.01%)	⬇️
postgres	79.31% <100.00%> (-0.01%)	⬇️
presto	54.08% <100.00%> (+<0.01%)	⬆️
python	83.37% <100.00%> (-0.01%)	⬇️
sqlite	77.89% <100.00%> (-0.01%)	⬇️
unit	55.06% <100.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
superset/daos/dashboard.py	96.71% <ø> (-0.05%)	⬇️
superset/models/embedded_dashboard.py	100.00% <100.00%> (ø)

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[michael-s-molina]

LGTM. Thanks for the improvement @john-bodley!