chore: Enable Ruby 3.4 CI builds

[jsonbailey]

Note

Adds Ruby 3.4 to CI builds and adds bigdecimal as a dev dependency for DynamoDB SDK.

• CI:
  • Expand Linux matrix in .github/workflows/ci.yml to include Ruby 3.4 alongside 3.2 and jruby-9.4.
• Dev Dependencies:
  • Add bigdecimal ~> 3.1.1 to launchdarkly-server-sdk.gemspec (required by aws-sdk-dynamodb).

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