chore: modernize

[mccutchen]

The results of running

go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

See modernize analyzer docs for more info.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.51%. Comparing base (d2a914c) to head (c56beb7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #221   +/-   ##
=======================================
  Coverage   94.51%   94.51%           
=======================================
  Files          10       10           
  Lines        2244     2244           
=======================================
  Hits         2121     2121           
  Misses         86       86           
  Partials       37       37           

Files with missing lines	Coverage Δ
httpbin/cmd/cmd.go	89.56% <100.00%> (ø)
httpbin/handlers.go	99.16% <100.00%> (ø)
httpbin/helpers.go	96.19% <100.00%> (ø)

🚀 New features to boost your workflow:

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