There are no Breaking Changes listed for JUCE 7.0.3, but I just found one. Thread::startThread (int priority) has been replaced by Thread::startThread (Priority threadPriority).
In 7.0.2 and before, the int priority parameter could be in the range from 0 - 10. Comments for the method noted:
Launches the thread with a given priority, where 0 = lowest, 10 = highest.
If the thread is already running, its priority will be changed.
Now the int parameter has been replaced by a Priority value. Because it’s an enum class, it’s not implicitly convertible to its integer value, but even if it was (or you used static_cast to convert it), the new integer range is [-2, 2], rather than [0, 10].