[core] Bump Next.js to 15.0.3

[hollandjake]

• Allowing support for next 15 and next 14 peer dependency

closes #4364

• I've read and followed the contributing guide on how to create great pull requests.
• I've updated the relevant documentation for any new or updated feature.
• I've linked relevant GitHub issue with "Closes #".
• I've added a visual demonstration in the form of a screenshot or video.

[Janpot]

The build passes now. @hollandjake @alexsenseer If you want to test drive this version and verify it works under Next.js 15:

pnpm add https://pkg.csb.dev/mui/toolpad/commit/8b8b3387/@toolpad/core

[hollandjake]

@Janpot all working fine in my project, i've been using the 0.8.1 version with next15 for a few weeks and not faced any issues

[hollandjake]

bumping docs and examples can be done in another PR to keep this one small and get it out there

[bharatkashyap]

I think it might be important to test out the examples with Next.js 15 before we merge this. I can get on that and report