Tune CI: modernize Perl workflow and add Dependabot config

[oalders]

Summary

• Dependabot (.github/dependabot.yml): new file. Weekly GitHub Actions updates with a 7-day cooldown, grouped into two rolling PRs (major-updates and minor-and-patch) so coordinated major bumps land atomically while routine bumps stay batched.
• CI workflow (.github/workflows/dzil-build-and-test.yml): six modernization transforms, one commit each, applied via the kitchen-sink:tune-perl-ci skill:
  1. fail-fast: false on every matrix job
  2. Extend Linux + macOS matrices through Perl 5.42
  3. Bump build/coverage to perldocker/perl-tester:5.42
  4. (no-op) push: was already restricted to master
  5. Workflow-level concurrency: cancel-in-progress block
  6. Standardize on install-with-cpm@v2 with App::cpm pin for Perls ≤ 5.22
  7. Drop macOS/Windows tests for Perls < 5.24

Test plan

• CI runs green on this PR (Linux matrix covers 5.10 → 5.42; macOS/Windows start at 5.24).
• Concurrency: pushing a new commit cancels the prior in-flight run.
• Dependabot opens a single minor-and-patch PR (not a flurry) on the next weekly tick.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.43%. Comparing base (99406a5) to head (7087e4c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   62.18%   62.43%   +0.24%     
==========================================
  Files           4        4              
  Lines         402      402              
  Branches      108      108              
==========================================
+ Hits          250      251       +1     
  Misses        104      104              
+ Partials       48       47       -1     

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