Fix dashboard XML: switch from <dashboard version=2> to Simple XML

The previous header (`<dashboard version="2" theme="dark">`) declared
Dashboard Studio (JSON definition format) while the body used Simple XML
elements (<row>/<panel>/<single>/<chart>/<table>). Splunk Web reported
"Layout undefined is not defined" because version=2 expects a JSON
<definition> block. Removed the version + theme attributes; the body is
already valid Simple XML. Dark theme is a per-user Splunk Web preference,
not a dashboard XML attribute.

Author: JonathanSolvesProblems

splunk_app/agentgate/default/data/ui/views/agentgate_audit.xml

@@ -1,4 +1,4 @@
-<dashboard version="2" theme="dark">
+<dashboard>
   <label>AgentGate Audit</label>
   <description>Every AI-agent action against Splunk that AgentGate has gated. Filter by decision, agent, or policy to investigate.</description>