feat(gantt): update styles

[Mister-Hope]

📑 Summary

Brief description about the content of your PR.

1. Removed duplicate css definition
2. Fix the nesting css definition as current support coverage is too low.
3. Apply titleText (which is used in other charts) to gantt chart
4. A little reformat and comment removement

Resolves #

📏 Design Decisions

Describe the way your implementation works or what design decisions you made if applicable.

📋 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.
• 🔖 targeted develop branch

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[codecov]

Codecov Report

Merging #4930 (1fec55d) into develop (12a4707) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4930   +/-   ##
========================================
  Coverage    79.97%   79.97%           
========================================
  Files          164      164           
  Lines        13623    13623           
  Branches       693      693           
========================================
  Hits         10895    10895           
  Misses        2579     2579           
  Partials       149      149           

Flag	Coverage Δ
e2e	85.26% <ø> (ø)
unit	43.29% <ø> (ø)

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

Files	Coverage Δ
packages/mermaid/src/diagrams/gantt/styles.js	100.00% <ø> (ø)