Remove digest dependency from gemspec

[iMacTia]

What are you trying to accomplish?

digest is a standard gem and as such is bundled with Ruby 3.0+, while in previous Ruby versions it's part of stdlib.
Having the gem explicitly added to the gemspec can cause issues downstream on projects using older ruby versions.
This is because it's actually possible to load the code twice, once from the stdlib and once from the gem.

As a reminder, the dependency was added as a result of this comment.

What approach did you choose and why?

Removing the dependency should have absolutely no effect on the code.
For people running Ruby 2.7 or previous, the require "digest" in cache.rb will load the stdlib file, while for those running Ruby 3.0+ it will load the standard gem.

What should reviewers focus on?

Cross-compatibility with Ruby 2 and Ruby 3

Type of Change

• Bugfix
• New feature
• Non-breaking change (a change that doesn't alter functionality - i.e., code refactor, configs, etc.)

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• It is safe to rollback this change.

cc @rafaelfranca @alexevanczuk

[iMacTia]

CLA is signed, not sure how to re-run the CI now though 😄

[alexevanczuk]

This looks fine to me, thanks @iMacTia !

[gmcgibbon]

while for those running Ruby 3.0+ it will load the standard gem.

> gem uninstall digest
Gem digest-3.1.0 cannot be uninstalled because it is a default gem

Hm, so digest is a default gem that can't be uninstalled. TIL. Seems kind of confusing that you can load default gems twice, but I guess Ruby 2 has no knowledge of this new paradigm and since the gemspec of digest supports Ruby 2, it just installs it like any other gem.

[gmcgibbon]

Thanks!