Added a release date format check to the linter

[kpumuk]

Historically, releases.yml included dates of the Ruby releases in a format compatible with YAML decoder. In the past few releases, specifically 3.2.9, 3.3.9 and 3.3.10, dates are encoded as strings. Loading the file now with permitted_classes: [Date] results in a date being a string instead of Date object for just the 3 releases mentioned.

This merge request introduces a linter check to catch such discrepancies:

$ bundle exec rake lint
Checking markdown files...
Ruby 3.2.9 release data (in `_data/releases.yml')
  release date is a string, not a Date object
Ruby 3.3.10 release data (in `_data/releases.yml')
  release date is a string, not a Date object
Ruby 3.3.9 release data (in `_data/releases.yml')
  release date is a string, not a Date object

[kpumuk]

@nagachika as you were the one introducing 3.3.10 (thank you!), I would appreciate any input on this MR.

[nagachika]

@kpumuk Thank you for your kind mention and adding a linter rule! I didn't notice about the issue at all.