[UX] Remove uppercase transform from fieldset legends in Basis

[klonos]

This is a follow-up of #3595.

In Seven we've removed the capitalization from the fieldset legend and summary (#4125). Should we do the same with Basis? Before/after screenshot:

[klonos]

...noting that we're removing the upper case from the formatting options fieldsets specifically in #3595. This issue here is for ALL the fieldsets in the rest of the theme.

[bugfolder]

Should we do the same with Basis?

Yes. All-caps is way less readable than mixed case.

[findlabnet]

WHEN YOU WRITE IN ALL CAPS IT SOUNDS LIKE YOU'RE SHOUTING.

From many sources.