[slider] Remove extra TalkBack focus targets

[atomiks]

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

Fixes #2575

Removing the tabIndex={-1} from both wrappers makes TalkBack land on the actual input[type="range"] only, which is the element that correctly handles increase/decrease actions

Preview: https://deploy-preview-4242--base-ui.netlify.app/react/components/slider

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/@base-ui/react@4242
  

  pnpm add https://pkg.pr.new/@base-ui/utils@4242
  

commit: badbf23

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+4B(0.00%)	🔺+11B(+0.01%)

Details of bundle changes

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[mj12albert]

In Firefox:

• This PR: pointer always triggers the focus visible style
• master: pointer never triggers focus visible, but keyboard after pointer (incorrectly) never triggers focus visible

Since Firefox is weird either way it's probably OK if it fixes the issue for TalkBack

[atomiks]

This PR: pointer always triggers the focus visible style

Not always, only if you've never interacted with another focusable element before. (Same as the issue we had some time ago.)

That said, FF supports focusVisible in .focus() broadly, so we can actually just use that 👍