fix: require date in Ruby 2.7

[exoego]

follow up #261
Ref. https://github.com/exoego/rspec-openapi/actions/runs/17423368635/job/49465804274

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (fc37deb) to head (a753fae).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #262   +/-   ##
=======================================
  Coverage   97.26%   97.26%           
=======================================
  Files          20       20           
  Lines         658      659    +1     
  Branches      148      148           
=======================================
+ Hits          640      641    +1     
  Misses         18       18           

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