appdata: exclude <name> element inside <developer>

<developer_name> has been deprecated in favor of <developer> with a
<name> child. We need to ensure that this developer name isn't parsed
as the application name.

Fixes: #5700
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f