[Select][NativeSelect] Polish CSS classes

[m4theushw]

Breaking changes

• [Select] The selectMenu slot was merged into select because selectMenu slot was redundant. The root slot is no longer applied to the select, but to the root.

  -<Select classes={{ root: 'class1', select: 'class2', selectMenu: 'class3' }} />
  +<Select classes={{ root: 'class1', select: 'class2 class3' }} />

• [NativeSelect] The selectMenu slot was merged into select because selectMenu slot was redundant. The root slot is no longer applied to the select, but to the root.

  -<NativeSelect classes={{ root: 'class1', select: 'class2', selectMenu: 'class3' }} />
  +<NativeSelect classes={{ select: 'class1 class2 class3' }} />

• [TablePagination] The classes.input key was changed to be applied on another element. Use classes.select instead.

  <TablePagination
  - classes={{ input: 'foo' }}
  + classes={{ select: 'foo' }}
  />

---

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

Part of #20012
Closes #11646
Closes #25313

I couldn't understand very well the discussion that lead to this change. I found #25766 (comment) about removing root and select. But the TODO in the code was to remove selectMenu. If it was not the specificity bug raised in #25763, we would only need to keep root.

[mui-pr-bot]

Details of bundle changes (experimental)

Generated by 🚫 dangerJS against edd965d

[oliviertassinari]

Almost good to go from my end

[oliviertassinari]

We could add a comment on why we bump the specificity, from what I remember, it's important to guarantee that the customization is applied. The base style comes from the InputBase-input which we have no injection order guarantee with.

Ah yes, we say "// Win specificity over the input base" below

[oliviertassinari]

We need to keep the bump:

Suggested change

	[`&.${selectClasses.select}`]: {
	// Win specificity over the input base
	'&': {

[oliviertassinari]

@m4theushw what do you think about this comment?

[m4theushw]

Updated. Missed this one because there's already a comment below.