OpenThread-BR: use correct ipv6 configuration

[tomfrenzel]

✍️ Description

This PR fixes the 100% CPU Load issue with the OpenThread-BR script.
The issue was that net.ipv6.conf.all.forwarding=1 disabled Route Advertisement. By default PVE should have net.ipv6.conf.all.accept_ra set to 1 which therefore gets passed into the LXC. But when enabling forwarding, RA gets disabled unless it is specifically set to 2.
This prevented otbr from noticing that thread nodes switched IP addresses and thus trying to reach them in a forever loop, causing the high CPU load.

Additionally, i added the accept_ra_rtr_pref and accept_ra_rt_info_max_plen parameters since those are documented as a requirement for OTBR to handle RA correctly.

I also added all 4 parameters for the interfaces all, default and eth0. The logic when all gets used / will be overidden is a bit tricky so i added eth0 (default when creating the LXC) to make sure the parameters always gets used.
The default interface defines that those parameters will also be used for new (currently not existing) interfaces. So if a user adds a second NIC to the LXC, it will use those parameters as well.

All those parameters are namespaced so it won't affect the host kernel (both privileged and unprivileged) 😉

Here are the docs tho the parameters i added:
https://openthread.io/codelabs/openthread-border-router#ping-from-a-linuxmacos-host

🔗 Related Issue

Fixes #14621

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to script metadata (PocketBase/website data).
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.