fix: 'YYYY-MM-DD' as gantt default date format

[good-jinu]

📑 Summary

The Gantt chart diagram type documentation claims that "The default input date format is YYYY-MM-DD."

See document

However in actuality, an explicit dateFormay YYYY-MM-DD must be given for such dates to be recognized.

Resolves #5862 , #5655

📏 Design Decisions

I set default date format as "YYYY-MM-DD"

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Latest commit: a8dfc66

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commit: a8dfc66

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 4.48%. Comparing base (df636c6) to head (a8dfc66).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6163   +/-   ##
=======================================
  Coverage     4.48%   4.48%           
=======================================
  Files          383     382    -1     
  Lines        54038   54027   -11     
  Branches       596     621   +25     
=======================================
  Hits          2425    2425           
+ Misses       51613   51602   -11     

Flag	Coverage Δ
unit	4.48% <100.00%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
packages/mermaid/src/diagrams/gantt/ganttDb.js	77.47% <100.00%> (ø)

... and 1 file with indirect coverage changes

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Build	Status	Details	Updated (UTC)
default (Inspect)	⚠️ Changes detected (Review)	339 added, 339 removed, 4 failures	Jan 3, 2025, 1:02 PM