use built-in concurrency configuration for duplicate workflows

[zacharyburnett]

In regards to cancelling duplicate workflows, this PR replaces the cancel_workflows.yml workflow with the built-in concurrency block.

This is:

1. faster
2. reduces the amount of workflows in the Actions queue, and
3. reduces our dependence on third-party workflow actions

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (a800608) 65.22% compared to head (fa5c284) 65.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   65.22%   65.22%           
=======================================
  Files         101      101           
  Lines        5450     5450           
=======================================
  Hits         3555     3555           
  Misses       1895     1895           

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[braingram]

Thanks! Much cleaner.