chore: add CODEOWNERS file

[kinyoklion]

Summary

Adds a CODEOWNERS file assigning @launchdarkly/team-sdk-java as the default code owners, consistent with other Java SDK repositories (java-core, android-client-sdk).

Link to Devin session: https://app.devin.ai/sessions/37fa817bb3614d8c8c8dd9de452ccc39
Requested by: @kinyoklion

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Note

Low Risk
Repository metadata only; no code, build, or runtime impact.

Overview
Adds a root CODEOWNERS file so @launchdarkly/team-sdk-java is the default owner for all paths (*), matching other Java SDK repos.

This only affects GitHub review routing and ownership metadata—no application or SDK runtime behavior changes.

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