[core] Avoid !important in borderColor prop

[rogerclotet]

• I have followed (at least) the PR section of the contributing guide.

• Avoid adding !important to the style rule when using borderColor.

Closes #16853

[mui-pr-bot]

Details of bundle changes.

Comparing: 14e595f...da436d1

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	-0.03%	-0.03%	329,716	329,629	90,142	90,117
@material-ui/core/Paper	0.00%	0.00%	68,673	68,673	20,470	20,470
@material-ui/core/Paper.esm	0.00%	0.00%	62,058	62,058	19,209	19,209
@material-ui/core/Popper	0.00%	0.00%	29,185	29,185	10,434	10,434
@material-ui/core/Textarea	0.00%	0.00%	5,759	5,759	2,368	2,368
@material-ui/core/TrapFocus	0.00%	0.00%	3,834	3,834	1,617	1,617
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,389	16,389	5,824	5,824
@material-ui/core/useMediaQuery	0.00%	0.00%	3,221	3,221	1,312	1,312
@material-ui/lab	0.00%	0.00%	152,604	152,604	46,521	46,521
@material-ui/styles	0.00%	0.00%	51,390	51,390	15,286	15,286
@material-ui/system	-0.55%	-0.68%	15,753	15,666	4,380	4,350
Button	0.00%	0.00%	79,426	79,426	24,280	24,280
Modal	0.00%	0.00%	15,050	15,050	5,233	5,233
Portal	0.00%	0.00%	3,579	3,579	1,568	1,568
Rating	0.00%	0.00%	70,735	70,735	22,079	22,079
Slider	0.00%	0.00%	75,071	75,071	23,272	23,272
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	52,044	52,044	13,816	13,816
docs.main	0.00%	0.00%	590,655	590,655	188,786	188,786
packages/material-ui/build/umd/material-ui.production.min.js	-0.03%	-0.04%	299,810	299,723	86,177	86,144

Generated by 🚫 dangerJS against da436d1

[eps1lon]

@rogerclotet Thanks.