Bump the version up to v0.15.0

[c-bata]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

None

What does this implement/fix? Explain your changes.

Bump the version up to v0.15.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.60%. Comparing base (652af58) to head (927124e).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #836   +/-   ##
=======================================
  Coverage   69.60%   69.60%           
=======================================
  Files          35       35           
  Lines        2375     2375           
=======================================
  Hits         1653     1653           
  Misses        722      722           

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[HideakiImamura]

LGTM.