Remove the deprecated displayname field from Dashboard extensions

[AlanGreene]

Changes

The displayname field was deprecated in v0.49.0 LTS.

Remove the remaining code that was added for backwards compatibility.

/kind misc

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Release Notes

Action required: The `displayname` field in Dashboard extensions is no longer supported, please rename to `displayName`. It was first deprecated in v0.49.0 LTS released in July 2024 and support is now removed.

[briangleeson]

/lgtm

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