Use default calibration and refine the logical-noise PEC convergence plot

[Henry Zou (henryzou50)]

This follow-up to #5616 removes the custom calibration_id and its outdated explanatory note so the tutorial uses the backend’s default calibration. It also narrows the convergence plot’s y-axis limits from ±0.18 to ±0.08, as Caleb suggested.

Current status

• All eight repository quality checks passed.
• The notebook still requires xslow; default-calibration execution has not yet been validated.
• The plotting code is updated, but the saved convergence chart still uses the previous limits.

Before merging

• Confirm xslow is deployed on ibm_boston.
• Verify xslow executes using the default calibration without a custom calibration_id. Small hardware test passed per Caleb’s recommendation, a full experiment rerun is not required.
• Keep the plotting code consistent with the saved chart. The axis-limit adjustment is deferred until the chart can be regenerated from saved results.
• Obtain final review.

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[qiskit-bot]

One or more of the following people are relevant to this code:

• Henry Zou (@henryzou50)

[Caleb Johnson (caleb-johnson)]

LGTM