fixing plot_histogram when data is sparse

[Shivansh20128]

Summary

I have added a new parameter "ignore_under" in the plot_histogram function. It makes the function ignore all the values in the data dictionary that are below this value.
Documentation for the parameter is also added.
#Resolves #13066

Details and comments

This solves the sparsity problem in the histogram function as mentioned in the issue here.

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[coveralls]

Pull Request Test Coverage Report for Build 11493960266

Details

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• 24 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.02%) to 88.664%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/visualization/counts_visualization.py	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	1	92.24%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	4	92.73%
crates/qasm2/src/parse.rs	18	96.69%

Totals
Change from base Build 11481266566:	-0.02%
Covered Lines:	74893
Relevant Lines:	84468

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💛 - Coveralls