[ButtonGroup] Fix variant outlined always has primary color borders on hover

[ZeeshanTamboli]

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Fixes #29219

The border-color style is applied in the Button component and we don't need to have it in ButtonGroup. Since the hovering behaviour is such that it is not on the complete ButtonGroup but instead on individual buttons within button group.

Since in ButtonGroup the border-right-color and border-bottom-color for horizontal and vertical orientation respectively is transparent, while hovering we take the current color value.

I think this is a regression from v5.0.0-alpha.25 from PR #24775.

In v4, we had border color styles while hovering on both ButtonGroup and Button but the Button ones had high specificity. In v5, with the new styling solution, the grouped selector within root ($ .grouped) in ButtonGroup got the high specificity. But anyway I think we need not have border color styles in ButtonGroup.

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Before v5.0.0-alpha.25 : https://codesandbox.io/s/mui-buttongroup-color-bug-forked-xw8ke?file=/src/Demo.tsx (in v5.0.0-alpha.24)

Before fix: https://codesandbox.io/s/mui-buttongroup-color-bug-t37rv?file=/src/Demo.tsx
After fix: https://codesandbox.io/s/mui-buttongroup-color-bug-63351?file=/src/Demo.tsx

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against c01fd13

[mnajdova]

Looks good 👌