Remove the deprecated `SelectPanel` experimental component

[hectahertz]

Closes https://github.com/github/primer/issues/5440

This pull request removes the deprecated SelectPanel2 component and all related files, stories, styles, and documentation from the codebase. It also updates references to its mock data in other story files and cleans up exports that previously included SelectPanel2 types and components.

Changelog

New

Changed/Removed

Component removal and cleanup:

• Deleted the entire SelectPanel2 component and its documentation file (SelectPanel.docs.json), removing all code, props, and subcomponent definitions.
• Removed all story files and story-specific CSS for SelectPanel2, including SelectPanel.stories.tsx, SelectPanel.playground.stories.tsx, SelectPanel.features.stories.module.css, and SelectPanel.module.css.

Exports and usage cleanup:

• Removed SelectPanel and its related types from the exports snapshot to ensure they are no longer part of the public API.
• Removed import of mockData from SelectPanel2 in ActionBar.examples.stories.tsx and replaced its usage with a hardcoded value to eliminate dependency on the deleted component.

Rollout strategy

• [ ] Patch release
• [ ] Minor release
• [X] Major release; if selected, include a written rollout or migration plan
• [ ] None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• [X] Added/updated tests
• [X] Added/updated documentation
• [X] Added/updated previews (Storybook)
• [X] Changes are SSR compatible
• [X] Tested in Chrome
• [ ] Tested in Firefox
• [ ] Tested in Safari
• [ ] Tested in Edge
• [ ] (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: e4fc7bc464127425248d10cbd642664eb8a74c2d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Major
@primer/mcp	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions[bot]]

:wave: Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions[bot]]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	87.97 KB (0%)
packages/react/dist/browser.umd.js	88.08 KB (0%)

[github-actions[bot]]

:wave: Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[lesliecdubs]

👋🏻 @hectahertz is this PR still on your radar to get to merge? I'm noticing an unanswered review question and a growing number of conflicts 👀

[hectahertz]

@francinelucca This is still pending on the migration from the feature teams (PRs, copilot)

https://primer-query.githubapp.com/?query=name%3A%22SelectPanel%22+entrypoint%3A%22%40primer%2Freact%2Fexperimental%22

I just updated the PR but it will fail again eventually.

At this time there's stiill 3 usages; we should decide now if we want to move it to dotcom instead or spend some resources on helping them finally migrate away.

Note that this won't get released until our next major release anyway.

cc @hellojanehere @lesliecdubs