[TextField] Reduce specificity of notchedOutline

[romanr]

Fixes #16287

Credit to @oliviertassinari

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

[mui-pr-bot]

Details of bundle changes.

Comparing: ac39205...bdf624e

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	-0.01%	0.00%	317,872	317,849	87,513	87,513
@material-ui/core/Paper	0.00%	0.00%	68,279	68,279	20,366	20,366
@material-ui/core/Paper.esm	0.00%	0.00%	61,574	61,574	19,162	19,162
@material-ui/core/Popper	0.00%	0.00%	28,945	28,945	10,399	10,399
@material-ui/core/Textarea	0.00%	0.00%	5,513	5,513	2,373	2,373
@material-ui/core/TrapFocus	0.00%	0.00%	3,753	3,753	1,577	1,577
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,009	16,009	5,793	5,793
@material-ui/core/useMediaQuery	0.00%	0.00%	2,597	2,597	1,100	1,100
@material-ui/lab	0.00%	0.00%	140,036	140,036	43,403	43,403
@material-ui/styles	0.00%	0.00%	51,699	51,699	15,346	15,346
@material-ui/system	0.00%	0.00%	15,420	15,420	4,393	4,393
Button	0.00%	0.00%	84,297	84,297	25,723	25,723
Modal	0.00%	0.00%	14,107	14,107	5,103	5,103
Portal	0.00%	0.00%	3,473	3,473	1,572	1,572
Slider	0.00%	0.00%	74,702	74,702	23,253	23,253
colorManipulator	0.00%	0.00%	3,904	3,904	1,544	1,544
docs.landing	0.00%	0.00%	55,119	55,119	13,940	13,940
docs.main	0.00%	0.00%	648,864	648,864	204,532	204,532
packages/material-ui/build/umd/material-ui.production.min.js	-0.01%	-0.00%	291,191	291,168	83,354	83,351

Generated by 🚫 dangerJS against bdf624e

[oliviertassinari]

@romanr It's a great first pull request on Material-UI 👌🏻. Thank you for working on it!