[ENH] Add the option to choose number of connections shown with plot_sensor_connectivity

[Avoide]

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PR Description

Moved the hard-coded variable n_con to be one of the parameters for the function call. Added a default option, to make it compatible with previous usage.

Merge checklist

Maintainer, please confirm the following before merging:

• [ ] All comments resolved
• [ ] This is not your own PR
• [ ] All CIs are happy
• [ ] PR title starts with [MRG]
• [ ] whats_new.rst is updated
• [ ] PR description includes phrase "closes <#issue-number>"