chore: Use gh-actions/actions/persistent-stores

[keelerm84]

The latest consul image incorrectly handles a '$' in the key name.
This is causing failing tests since we use an '$inited' key. Until the
consul change is remedied, we need to pin to a working version
of consul.

This is already done in our shared GH action for persistent stores, so I
am updating this SDK to finally use that.

---

Note

Switches the build workflow to the shared persistent-stores action and makes a minor spec cleanup.

• CI:
  • Replace inline service containers with launchdarkly/gh-actions/actions/persistent-stores@persistent-stores-v0 in .github/workflows/build-gem.yml.
  • Enable redis, consul, and dynamodb via action inputs.
• Tests:
  • Simplify array join in spec/ldclient_spec.rb (flags_list.map { |f| f[:key] }.join(", ")).

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