[Skeleton] Make default CSS display mode to block

[ahtcx]

fixes #17405

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

[mui-pr-bot]

Details of bundle changes.

Comparing: 6a65aa7...a6a5e0a

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	331,787	331,787	90,602	90,602
@material-ui/core/Paper	0.00%	0.00%	68,813	68,813	20,494	20,494
@material-ui/core/Paper.esm	0.00%	0.00%	62,185	62,185	19,222	19,222
@material-ui/core/Popper	0.00%	0.00%	28,466	28,466	10,177	10,177
@material-ui/core/Textarea	0.00%	0.00%	5,082	5,082	2,133	2,133
@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%	16,410	16,410	5,838	5,838
@material-ui/core/useMediaQuery	0.00%	0.00%	2,558	2,558	1,066	1,066
@material-ui/lab	+0.01% 🔺	+0.01% 🔺	153,690	153,706	46,768	46,772
@material-ui/styles	0.00%	0.00%	51,508	51,508	15,308	15,308
@material-ui/system	0.00%	0.00%	15,668	15,668	4,361	4,361
Button	0.00%	0.00%	78,700	78,700	24,045	24,045
Modal	0.00%	0.00%	14,601	14,601	5,121	5,121
Portal	0.00%	0.00%	2,907	2,907	1,322	1,322
Rating	0.00%	0.00%	70,053	70,053	21,881	21,881
Slider	0.00%	0.00%	75,174	75,174	23,206	23,206
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	52,232	52,232	13,768	13,768
docs.main	0.00%	0.00%	597,448	597,448	190,763	190,763
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	302,596	302,596	86,953	86,953

Generated by 🚫 dangerJS against a6a5e0a

[oliviertassinari]

Thanks!