CI uses the latest mbed-beta core which is a placeholder release

[sebromero]

To "smoothly" phase out the users from using the beta track of the mbed core we chose to mark it deprecated. To make the users get a notification about this we bumped the version to 1.3.1. That version is a dummy release though and the CI wants to use it. Let's change the CI to use the release version of the mbed core.

@per1234
@facchinm

[sebromero]

Solved via 8966111

[per1234]

@sbhklr would you like me to take charge of updating the other CI workflows that use the arduino-beta:mbed core?

[sebromero]

@per1234 That would be great as it seems like they will also fail :-)