Fix Ruby 3.3 deprecation warning

[olegantonyan]

Fix this warning in Ruby 3.3

warning: observer was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add observer to your Gemfile or gemspec

[mike-burns]

#1608 - this is also my attempt to fix tests.

[mike-burns]

#1609 - still attempting to fix tests. Hold tight.

[mike-burns]

@olegantonyan do you mind rebasing on main? That ought to fix CI.

[mike-burns]

Let's get this out ASAP. I'm going to:

1. git rebase main
2. git push --force
3. git checkout main && git merge olegantonyan-main

Then I'll cut a release.

Thank you for this fast contribution!

[olegantonyan]

@mike-burns sure! thank you

[mike-burns]

I screwed something up but it's in as adc4857.