Add auditEventsStreamingHeadersDestroy Mutation

What does this MR do and why?

• Adds a new graphQL mutation auditEventsStreamingHeadersDestroy to destroy a custom header on a streaming destination.

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How to set up and validate locally

• Enable the streaming_audit_event_headers feature flag.
• Create an Ultimate group.
• Create a streaming destination. (Group -> Security & Compliance -> Audit Events -> Streams)
• Using GraphQL, create a header.

mutation {
  auditEventsStreamingHeadersCreate(input: { destinationId: "gid://gitlab/AuditEvents::ExternalAuditEventDestination/ID GOES HERE", key: "X-Foo", value: "bar" }) {
    errors
    clientMutationId
    header {
      id
      value
      key
    }
  }
}

• Using the returned ID value, delete the same header.

mutation {
  auditEventsStreamingHeadersDestroy(input: { headerId: "gid://gitlab/AuditEvents::Streaming::Header/7" }) {
    errors
    clientMutationId
  }
}

• Ensure that the header has been deleted from the database.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #361217 (closed)