Replace geom_bar(stat = identity) with geom_col()

[utkarshpawade]

Fixes #482

Summary
•Replaced geom_bar(stat = "identity") with geom_col() in plot_scheme() (bayesplot-colors.R) and mcmc_acf() (mcmc-diagnostics.R)
•geom_col() is the documented modern equivalent and removes redundant arguments (stat = "identity", position = "identity")

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (906cd5c) to head (de9127e).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
- Coverage   98.66%   98.66%   -0.01%     
==========================================
  Files          35       35              
  Lines        5860     5857       -3     
==========================================
- Hits         5782     5779       -3     
  Misses         78       78              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jgabry]

Thanks! Looks good, just one suggested edit to the NEWS file