[ExpansionPanelSummary] Convert to function component

[joshwooding]

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

Everything must be converted! 😁

[mui-pr-bot]

Details of bundle changes.

Comparing: 2e76f08...fed66ad

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	-0.01%	-0.05%	352,667	352,641	90,540	90,497
@material-ui/core/Paper	0.00%	0.00%	67,382	67,382	19,709	19,709
@material-ui/core/Paper.esm	0.00%	0.00%	61,143	61,143	18,799	18,799
@material-ui/core/Popper	0.00%	0.00%	30,460	30,460	10,527	10,527
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	17,384	17,384	5,727	5,727
@material-ui/core/useMediaQuery	0.00%	0.00%	2,469	2,469	1,038	1,038
@material-ui/lab	0.00%	0.00%	152,123	152,123	44,511	44,511
@material-ui/styles	0.00%	0.00%	52,565	52,565	15,330	15,330
@material-ui/system	0.00%	0.00%	17,138	17,138	4,519	4,519
Button	0.00%	0.00%	90,889	90,889	26,921	26,921
Modal	0.00%	0.00%	84,633	84,633	25,232	25,232
colorManipulator	0.00%	0.00%	3,232	3,232	1,300	1,300
docs.landing	0.00%	0.00%	51,883	51,883	11,372	11,372
docs.main	0.00%	0.00%	647,626	647,626	200,974	200,974
packages/material-ui/build/umd/material-ui.production.min.js	-0.00%	-0.03%	301,815	301,804	83,756	83,728

Generated by 🚫 dangerJS against fed66ad