Docs: Document required dependency for JsonToGrpc filter

[Ava051213]

What does this PR do?

Documents the required jackson-dataformat-protobuf dependency for the
JsonToGrpc filter.

Why is it needed?

Without this dependency, users get a NoClassDefFoundError when using the
JsonToGrpc filter.

Fixes #3424

[Copilot]

Pull request overview

This PR updates the Spring Cloud Gateway documentation to clarify the additional Jackson module required to use the JsonToGrpc GatewayFilter, preventing runtime NoClassDefFoundError due to a missing Protobuf JsonFactory.

Changes:

• Add a “Required Dependencies” section to the JsonToGrpc filter documentation.
• Document the com.fasterxml.jackson.dataformat:jackson-dataformat-protobuf dependency needed for JSON↔gRPC conversion.

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[Copilot]

This sentence refers to “dependencies” but only a single dependency is listed below. Update the wording to “dependency” to match the example.

Suggested change

	=== Required Dependencies
	To use the `JsonToGrpc` filter, add the following dependencies:
	=== Required Dependency
	To use the `JsonToGrpc` filter, add the following dependency:

[Copilot]

The section title is using a level-3 heading (===) even though this page doesn’t define any level-2 sections (==). That will nest this content oddly in the rendered docs/TOC. Use a level-2 heading (== Required Dependencies) instead (consistent with other GatewayFilter factory pages which don’t use === headings).

Suggested change

	=== Required Dependencies
	== Required Dependencies