Update GRPCRoute Core Support Level

[kate-osborn]

Proposed changes

Problem: The summary table in the Gateway API Compatibility document lists GRPCRoute Core as "Partially Supported"

Solution: Update GRPCRoute Core to "Supported" in the Gateway API Compatibility document

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

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