Fix barrier label when output='mpl'

[atharva-satpute]

Summary

Fixes #13609
Changed the way the barrier label is applied to qubits.

Details and comments

PR-13780 resolved the issue for output=text, but the labels remained incorrectly placed when output=mpl.

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[coveralls]

Pull Request Test Coverage Report for Build 13735840737

Details

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.005%) to 88.132%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/visualization/circuit/matplotlib.py	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	2	94.59%
crates/qasm2/src/lex.rs	6	92.48%

Totals
Change from base Build 13730247126:	-0.005%
Covered Lines:	72638
Relevant Lines:	82420

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💛 - Coveralls