Resolve TypeScript errors in React 19 (by removing the deprecated propTypes.ts)

[Venryx]

Removed the deprecated propTypes.ts, and some remaining references to its exports, in order to make mobx-react compatible with React 19 (without TypeScript compile errors), since React 19 removes its prop-types exports of React.Requireable and React.Validator. (@mweststrate gave a go-ahead for this here)

Changelog:

• Removed propTypes.ts, as React 19 no longer provides the types React.Requireable and React.Validator.
• Updated docs to not mention the mobx-react specific prop-types anymore.
• Removed the propTypes.test.ts file. (not applicable anymore, since the mobx-react specific prop-types no longer exist)
• Removed the "component with observable propTypes" test, since it's no longer applicable. (and it was referencing typescript types that are no longer importable, so using test.skip is not sufficient)

Code change checklist

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

[changeset-bot]

🦋 Changeset detected

Latest commit: ea92a46

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

This PR includes changesets to release 1 package

Name	Type
mobx-react	Major

Not sure what this means? Click here to learn what changesets are.

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

[Venryx]

I added a basic changelog entry for the PR: (to satisfy changeset-bot)

Removed the deprecated PropTypes export, as React 19 no longer provides the types React.Requireable and React.Validator, and this was causing TypeScript errors with @types/react@19.0.10.

I marked it as a major change (assuming even deprecated APIs are considered part of the semver-tracked api-surface), but I can change it to minor if needed.