power-profile-row: Use "subtitle" style class for power profile descriptions
requested to merge automerisnaranja/gnome-control-center:power-profiles-subtitles-ui-tweaks into main
Doing so, the power profiles rows will look consistent with other rows, such as the "Dynamic workspaces" and "Fixed number of workspaces" from the Multitasking panel. This cleans the code a bit.
Also, remove periods from the subtitles as they aren't necessary.
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