[2.x] Move gantt-chart out from subdirectory (#321)

[joshuali925]

Signed-off-by: Joshua Li [email protected]
(cherry picked from commit 12ad9b7)

Description

backport #321

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2a756cf) 94.73% compared to head (af334ca) 94.73%.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x     #323   +/-   ##
=======================================
  Coverage   94.73%   94.73%           
=======================================
  Files           5        5           
  Lines         152      152           
  Branches       45       45           
=======================================
  Hits          144      144           
  Misses          8        8           

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