Add `observer` to the SDK's gemspec to avoid warnings in ruby 3.3/support ruby 3.4

[dgollahon]

Describe the bug
I get the following warning in ruby 3.3.1 using sdk version 7.2.0 (but looks to still apply on master):

warning: [...]/lib/ruby/3.3.0/observer.rb 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. Also contact author of launchdarkly-server-sdk-7.2.0 to add observer into its gemspec.

To reproduce
Require the gem with $VERBOSE = true on ruby 3.3.x

Expected behavior
No warnings are emitted.

Logs
N/A

SDK version
7.2.0

Language version, developer tools
Ruby 3.3.1

OS/platform
MacOS

Additional context
N/A

[keelerm84]

Thank you for bringing this to our attention. I will make sure it gets addressed soon and will let you know once it has been resolved.

[keelerm84]

This has been resolved in 8.4.1

[dgollahon]

Thank you!