[CI/CD] Switch from self-hosted approach to runners offered by GitHub

[mfbrantner]

While the current CI/CD pipeline works, there are some issues that should be worked out.

• Since updating the Unity Editor version, the Build workflow takes about 1:20h to run.

• The self-hosted runner requires workarounds that are prone to change and tedious to maintain. It is also unsuited for parallel job execution.

Switching to runners hosted by GitHub has multiple benefits:

• Parallel execution of jobs, resulting in an overall faster workflow
• Resistant to failures due to their ephemeral nature
• (Almost) no workarounds required, thus decreasing workflow complexity
• Better network speeds
• Don't rely on TU Graz Power/Network infrastructure

I will submit a PR with the proposed changes shortly.