Use consistent "percent" naming in systemmonitor

[spacegaier]

Breaking change

Proposed change

Follow-up to PR #36283, specifically this request from @MartinHjelmare #36283 (comment)

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[spacegaier]

@MartinHjelmare I also considered removing the "(percent)" for all sensors, since it is clear from the "%" in the value. However, since the entity_id is derived from the name, the IDs would would look bad (sensor.disk_use_home and sensor.disk_use_home_2 instead of sensor.disk_use_home and sensor.disk_use_percent_home as they are today). When I tried setting the entity_id directly in the integration, it failed and I think that is anyway not the way to go, correct?

[MartinHjelmare]

This is good! Thanks!