[core] Switch from buttonRef to ref usage

[eps1lon]

With ref forwarding we don't need buttonRef anymore. Since we will deprecate this in some minor v4 release we should start using the preferred prop.

[mui-pr-bot]

Details of bundle changes.

Comparing: 650ddda...cbaebdc

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	347,585	347,585	89,756	89,756
@material-ui/core/Paper	0.00%	0.00%	68,404	68,404	20,052	20,052
@material-ui/core/Paper.esm	0.00%	0.00%	60,735	60,735	18,784	18,784
@material-ui/core/Popper	0.00%	0.00%	34,930	34,930	11,918	11,918
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	17,877	17,877	5,962	5,962
@material-ui/core/useMediaQuery	0.00%	0.00%	2,463	2,463	1,044	1,044
@material-ui/lab	-0.02%	-0.01%	148,083	148,047	43,916	43,913
@material-ui/styles	0.00%	0.00%	53,143	53,143	15,442	15,442
@material-ui/system	0.00%	0.00%	17,132	17,132	4,519	4,519
Button	0.00%	0.00%	88,669	88,669	26,422	26,422
Modal	0.00%	0.00%	82,688	82,688	24,801	24,801
colorManipulator	0.00%	0.00%	3,745	3,745	1,537	1,537
docs.landing	0.00%	0.00%	49,820	49,820	10,807	10,807
docs.main	0.00%	0.00%	644,591	644,591	200,685	200,685
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	296,244	296,244	82,780	82,780

Generated by 🚫 dangerJS against cbaebdc