[checkbox] Set aria-checked=mixed when indeterminate

[mj12albert]

Fixes #20476

Indeterminate checkboxes don't have any specific logic, it's completely user-controlled so we can simply add the attribute here

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Bundle	Parsed size	Gzip size
@mui/material	🔺+32B(+0.01%)	🔺+22B(+0.02%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

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[silviuaavram]

Do we need to set the aria attribute? Since we are using a native input type checkbox, I thought we don't need the aria-indeterminate.

For example, I tested with VO and Chrome, and I see no difference in reading. Both using and not using aria-indeterminate="mixed" and I get the same unchecked reading.

[mj12albert]

Do we need to set the aria attribute? Since we are using a native input type checkbox, I thought we don't need the aria-indeterminate.

For example, I tested with VO and Chrome, and I see no difference in reading. Both using and not using aria-indeterminate="mixed" and I get the same unchecked reading.

@silviuaavram Support is spotty, but proper implementations announce something like "partially checked", so it's a small nice to have I guess

AFAIK it works with JAWS+Chrome, Win Narrator+Edge

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Bundle size

Bundle	Parsed size	Gzip size
@mui/material	🔺+32B(+0.01%)	🔺+17B(+0.01%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

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Cherry-pick PRs will be created targeting branches: v7.x