Fix @babel/plugin-proposal-private-property-in-object errors

Looks like [@babel/plugin-proposal-private-property-in-object](https://www.npmjs.com/package/@babel/plugin-proposal-private-property-in-object) is deprecated and to be replaced with [@babel/plugin-transform-private-property-in-object](https://www.npmjs.com/package/@babel/plugin-transform-private-property-in-object).

Honestly I'm not sure how much we use it, so I'd be glad to not have this warning every time, but I haven't looked into it. Here's what warning we get when running a build:

```
Creating an optimized production build...
One of your dependencies, babel-preset-react-app, is importing the
"@babel/plugin-proposal-private-property-in-object" package without
declaring it in its dependencies. This is currently working because
"@babel/plugin-proposal-private-property-in-object" is already in your
node_modules folder for unrelated reasons, but it may break at any time.

babel-preset-react-app is part of the create-react-app project, which
is not maintianed anymore. It is thus unlikely that this bug will
ever be fixed. Add "@babel/plugin-proposal-private-property-in-object" to
your devDependencies to work around this error. This will make this message
go away.
```

Adding it in dev dependencies didn't make the warnings go away, maybe migrating over or removing this somehow might be the move.

Relevant links:

https://babeljs.io/docs/babel-plugin-transform-private-property-in-object

https://stackoverflow.com/questions/76435306/babel-preset-react-app-is-importing-the-babel-plugin-proposal-private-propert

No source file or test is named. Reproduce the warning with the production build, then inspect the package manifest and build configuration to determine whether the deprecated plugin is used directly or only through babel-preset-react-app. Done means the warning is gone and the production build still succeeds.

Written by the indexing model from the issue text.