[DialogTitle] Update default element from h6 to h2

[dayander]

Closes #16569

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

[mui-pr-bot]

Details of bundle changes.

Comparing: 76b7b7e...64ec6e6

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	327,648	327,663	90,287	90,288
@material-ui/core/Paper	0.00%	0.00%	68,477	68,477	20,407	20,407
@material-ui/core/Paper.esm	0.00%	0.00%	61,761	61,761	19,190	19,190
@material-ui/core/Popper	0.00%	0.00%	28,942	28,942	10,407	10,407
@material-ui/core/Textarea	0.00%	0.00%	5,505	5,505	2,365	2,365
@material-ui/core/TrapFocus	0.00%	0.00%	3,753	3,753	1,576	1,576
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,160	16,160	5,813	5,813
@material-ui/core/useMediaQuery	0.00%	0.00%	2,595	2,595	1,104	1,104
@material-ui/lab	0.00%	0.00%	138,027	138,027	42,562	42,562
@material-ui/styles	0.00%	0.00%	51,887	51,887	15,384	15,384
@material-ui/system	0.00%	0.00%	15,576	15,576	4,432	4,432
Button	0.00%	0.00%	81,161	81,161	24,779	24,779
Modal	0.00%	0.00%	14,451	14,451	5,078	5,078
Portal	0.00%	0.00%	3,471	3,471	1,573	1,573
Slider	0.00%	0.00%	75,100	75,100	23,309	23,309
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	54,338	54,338	13,762	13,762
docs.main	0.00%	0.00%	647,763	647,763	204,199	204,199
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	300,063	300,078	86,013	86,014

Generated by 🚫 dangerJS against 64ec6e6

[eps1lon]

@dayander Very much appreciated.

Waiting for reports that this broke because people used h7+ headings in their dialogs though 😄