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>

server/boards/services/telemetry/telemetry.go

@@ -17,7 +17,7 @@ import (
 )
 
 const (
-	rudderKey                  = "placeholder_rudder_key"
+	rudderKey                  = "placeholder_boards_rudder_key"
 	rudderDataplaneURL         = "placeholder_rudder_dataplane_url"
 	timeBetweenTelemetryChecks = 10 * time.Minute
 )