Replace FileList with Dir to fix error

This causes an error on bundle install when installing the gem from source. FileList comes from Rake but it's unnecessary as Dir is a very common use in popular gemspecs.

[!] There was an error while loading `launchdarkly-server-sdk.gemspec`: uninitialized constant FileList. Bundler cannot continue.

 #  from /Users/j.julio/.rbenv/versions/3.3.5/lib/ruby/gems/3.3.0/bundler/gems/launchdarkly-ruby-server-sdk-b8220416fe3f/launchdarkly-server-sdk.gemspec:18
 #  -------------------------------------------
 #  
 >    spec.files         = FileList["lib/**/*", "README.md", "LICENSE.txt"]
 #    spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
 #  -------------------------------------------

launchdarkly-server-sdk.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/launchdarkly/ruby-server-sdk"
   spec.license       = "Apache-2.0"
 
-  spec.files         = FileList["lib/**/*", "README.md", "LICENSE.txt"]
+  spec.files         = Dir["lib/**/*.rb", "README.md", "LICENSE.txt"]
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
   spec.required_ruby_version = ">= 3.0.0"