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"

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• 🦋 If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[changeset-bot]

🦋 Changeset detected

Latest commit: 02efa03

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
mermaid	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[netlify]

✅ Deploy Preview for mermaid-js ready!

Name	Link
🔨 Latest commit	02efa03
🔍 Latest deploy log	https://app.netlify.com/sites/mermaid-js/deploys/6795e934ffafe00008accabd
😎 Deploy Preview	https://deploy-preview-6163--mermaid-js.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[pkg-pr-new]

Open in Stackblitz

npm i https://pkg.pr.new/mermaid-js/mermaid@6163

npm i https://pkg.pr.new/mermaid-js/mermaid/@mermaid-js/layout-elk@6163

npm i https://pkg.pr.new/mermaid-js/mermaid/@mermaid-js/mermaid-zenuml@6163

npm i https://pkg.pr.new/mermaid-js/mermaid/@mermaid-js/parser@6163

commit: 02efa03

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 4.47%. Comparing base (e2de552) to head (02efa03).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #6163      +/-   ##
==========================================
- Coverage     4.47%   4.47%   -0.01%     
==========================================
  Files          384     385       +1     
  Lines        54179   54190      +11     
  Branches       623     598      -25     
==========================================
  Hits          2425    2425              
- Misses       51754   51765      +11     

Flag	Coverage Δ
unit	4.47% <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

[argos-ci]

The latest updates on your projects. Learn more about Argos notifications ↗︎

Awaiting the start of a new Argos build…