update project template

This allows us to use target_compile_features to set the C++ standard
in cppext and move to a newer standard.

The stubgen version on bookworm is too old to know the docstrings option.
The output is not particularly useful, but it allows the build chain to run through.

std::format is not available on bookworm yet. It was included but not used.

When terminating the application while poll-type inputs are waiting
for initial values from other ApplicationModules, we need to interrupt()
those blocking initial-value reads.

This requires a corresponding change in DeviceAccess to allow calling
interrupt() without wait_for_new_data.

They have not been touched for ages and are now broken

fix(project-template): (re-)enable DOT support in Doxygen

libxml2 will print out a confusing "I/O warning" if we try to open a
non-existant XML file, and the only way to suppress it is marked as
"DON'T USE" so we check beforehand now

Accessing the model in the ApplicationModule's main loop is not thread safe.

Closes #379

If the return channel of a FeedingFanOut was removed because it is
connected to the control system but not mapped there, the _returnSlave
is not set and hence all read operations on it were crashing.

fix(project-template): find pattern in coding style check

We were holding an std::vector of an incomplete type
(VariableNetworkNode) in a struct (ProcessVariableProperties), which
is not allowed. This fix changes it into an std::vector of shared_ptrs.
Since we are exposing this vector via ProcessVariableProxy::getNodes(),
this presents a breaking API change. Since this is used mostly
internally, this should not break much.