ci: relax span-start-finish-traceid128 SLO (#14917)

## Description

In PR #14773 we improved span start overhead and reduced the SLOs, but
we may have been too aggressive with lowering the
span-start-finish-traceid128 SLO.

## Testing

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## Risks

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-->

## Additional Notes

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Author: brettlangdon

.gitlab/benchmarks/bp-runner.microbenchmarks.fail-on-breach.yml

@@ -1067,7 +1067,7 @@ experiments:
               - max_rss_usage < 34.00 MB
           - name: span-start-finish-traceid128
             thresholds:
-              - execution_time < 56.00 ms
+              - execution_time < 57.00 ms
               - max_rss_usage < 34.00 MB
           - name: span-start-traceid128
             thresholds: