Boards: Use custom placeholder for telemetry. (#22623)

Adopt `placeholder_boards_rudder_key` as the replacement value for injecting the telemetry key, allowing Boards to preserve its unique telemetry key.

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/boards/src/index.tsx

@@ -85,7 +85,7 @@ function getSubpath(siteURL: string): string {
     return url.pathname.replace(/\/+$/, '')
 }
 
-const TELEMETRY_RUDDER_KEY = 'placeholder_rudder_key'
+const TELEMETRY_RUDDER_KEY = 'placeholder_boards_rudder_key'
 const TELEMETRY_RUDDER_DATAPLANE_URL = 'placeholder_rudder_dataplane_url'
 const TELEMETRY_OPTIONS = {
     context: {