Add dependencies on gems that are no longer default in Ruby 3.5.

[mattmccutchen]

Without this, main would fail to load those gems in Ruby 3.5. Resolves a warning in Ruby 3.3.5 and 3.4.

---

When using Braid with Ruby 3.3.5, I got the warnings:

/home/user/braid/braid-1.wt/lib/braid/main.rb:14: warning: logger was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add logger to your Gemfile or gemspec to silence this warning.
/home/user/braid/main.gitwt/lib/main.rb:80: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.

(The first line number refers to a line in Braid that says require 'main'. I don't know why the warning is blamed on that line rather than on the require 'logger' line in main.)

So I added the explicit dependencies to the gemspec. Ideally, they would have meaningful minimum versions, but researching what the minimum versions should be looks like a time sink to me. If main declared a minimum Ruby version, then it would be reasonable to just take the gem versions from that Ruby version, but since it doesn't, the risk of allowing a too-old version of logger or ostruct doesn't seem fundamentally worse to me than the risk of allowing a too-old Ruby.

The warnings are reported only when running under Bundler (edit 2024-11-04; I'll be happy to update the commit message if/when there's interest in merging this PR). The presence of the warnings for logger and ostruct in Ruby 3.3.5 is considered a bug, and those warnings will be removed in Ruby 3.3.6. But Ruby 3.4 is expected soon enough (2024-12-25) that I think it's reasonable to go ahead and make this change to reduce the annoyance on Ruby 3.3.5 too.

I looked for documentation about these warnings and didn't find any; I only found the code in Ruby that generates them in case that's helpful.