deprecate ubuntu-fan in Noble

[Impact]

In order to provide ubuntu-fan we need to maintain additional kernel SAUCE patches that are currently conflicting with upstream code, potentially breaking networking eBPF APIs.

To prevent such incompatibility the whole patch set requires a major redesign.

However, after investigations and a detailed assessment we have not found any relevant real-world use-case that still requuires ubuntu-fan, as most of its features are now available in the upstream kernel and user-space tools, using alternative solutions.

Moreover, maintaining ubuntu-fan is also slowing down / preventing the development of other packages (see for example LP: #2051672).

Therefore, we are proposing to deprecate ubuntu-fan starting with noble.

[Test case]

We have ubuntu-fan test cases in our regression testing suite. Such tests are expected to fail with the noble kernel, so we can hint/disable them in Noble.

[Fix]

Drop (do not apply) the ubuntu-fan SAUCE patch set from the Ubuntu kernel.

[Regression potential]

There are still some existing tools/systems that may still rely on ubuntu-fan, so we may experience regressions when such systems are moving to noble. However, the potential of breaking eBPF networking has a much higher impact, so we can probably workaround any potential ubuntu-fan regression using alternative upstream solutions.