Upgrade to Unity 6 [MTT-9275]

[FabianStoll]

Description

This PR upgrades Boss Room to the newly released Unity 6 LTS, specifically version 6000.0.25f1.
It also includes an upgrade of the Netcode for GameObjects package to version 2.0.0.

With the release of Unity 6 the Multiplayer Play Mode package is now available to simulate up to four virtual players in the Editor for better multiplayer testing. We have replaced ParrelSync with Multiplayer Play Mode.

Issue Number(s)

MTT-9275
MTT-8706

Contribution checklist

• Tests have been added for boss room and/or utilities pack
• Release notes have been added to the project changelog file and/or package changelog file
• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• JIRA ticket ID is in the PR title or at least one commit message
• Include the ticket ID number within the body message of the PR to create a hyperlink
• An Index entry has been added in readme.md if applicable

[unity-cla-assistant]

All committers have signed the CLA.

[fernando-cortez]

Minor changes to make!
I had a few warnings when I opened the project for deprecated API. Those should probably be addressed with this upgrade -- I'll send those offline.

[fernando-cortez]

Looks good to me!
I created a task to go through the rest of the warnings.
Let's wait to hear back from QA on the Android test.

[fernando-cortez]

Let's make a ticket to bump up the editor version when it's okay to do so.
Otherwise, good to go!