Do not to run apt CI tests on Debian Sid

[traversaro]

Debian Testing should anyhow catch problems in future Ubuntu releases, but with less instabilities than Debian Sid.

Fix https://github.com/robotology/idyntree/issues/1100 .

[traversaro]

Now the build fails:

2023-09-01T11:47:53.5190452Z /__w/idyntree/idyntree/ycm/build/yarp/src/libYARP_serversql/src/yarp/serversql/impl/ConnectThread.h:24:10: error: 'string' in namespace 'std' does not name a type
2023-09-01T11:47:53.5190905Z    24 |     std::string src;
2023-09-01T11:47:53.5191118Z       |          ^~~~~~
2023-09-01T11:47:53.5191826Z /__w/idyntree/idyntree/ycm/build/yarp/src/libYARP_serversql/src/yarp/serversql/impl/ConnectThread.h:13:1: note: 'std::string' is defined in header '<string>'; did you forget to '#include <string>'?
2023-09-01T11:47:53.5192244Z    12 | #include <mutex>
2023-09-01T11:47:53.5192461Z   +++ |+#include <string>
2023-09-01T11:47:53.5192663Z    13 | 
2023-09-01T11:47:53.5193700Z /__w/idyntree/idyntree/ycm/build/yarp/src/libYARP_serversql/src/yarp/serversql/impl/ConnectThread.h:25:10: error: 'string' in namespace 'std' does not name a type
2023-09-01T11:47:53.5194108Z    25 |     std::string dest;
2023-09-01T11:47:53.5194315Z       |          ^~~~~~
2023-09-01T11:47:53.5194965Z /__w/idyntree/idyntree/ycm/build/yarp/src/libYARP_serversql/src/yarp/serversql/impl/ConnectThread.h:25:5: note: 'std::string' is defined in header '<string>'; did you forget to '#include <string>'?
2023-09-01T11:47:53.5195390Z    25 |     std::string dest;
2023-09-01T11:47:53.5195593Z       |     ^~~

This either requires a fix in YARP or updating the YARP version.

[traversaro]

As there is no immediate fix on iDynTree's side, let's just remove the debian:sid job that anyhow it was just a nice to have feature.