[improve][docs] Clarify Java client setup and Functions runtime requirements

[lhotari]

Java client setup now links to a dedicated dependency configuration page with copyable Maven and Gradle builds. The guide explains the single unshaded dependency for the v4, v5, and admin clients, aligns Pulsar and Netty with BOMs, and documents exclusions, the shaded fallback, Spring Boot, and Gradle API/implementation separation.

Also:

• Add a version-aware upgrade tip to the documentation portal and link release highlights to the Java dependency guide.
• Add trusted-code warnings to Functions worker and runtime guides, and remove the obsolete Python protobuf installation instruction.
• Refresh the 5.0.x documentation snapshot from the current docs; the regenerated version-specific sidebar is unchanged.

Validation:

• Built the site with both current and 5.0.x documentation enabled; existing unrelated link warnings remain.
• Compiled sample code against the v4, v5, and admin APIs using the documented Maven and Gradle builds, including the shaded fallback.
• Verified that a Gradle consumer receives the public API artifacts without client implementations on its compile classpath.
• Confirmed all 259 files in the 5.0.x snapshot match the current docs and ran git diff --check.