Update JUCE submodule from 7.0.3 to 8.0.12 for macOS 15+ compatibility

[jinpavg]

this should close #24

had to bump min CMake version and min C++ version in CMakeTexts.txt to make JUCE 8 happy

also updated MainComponent.cpp with the replacement functions JUCE 8 recommends for their deprecated MidiInput::getDevices(), AudioDeviceManager::setMidiInputEnabled(), and AudioDeviceManager::addMidiInputCallback()

[jinpavg]

There are some other deprecation warnings too, but they don't break the template.

Claude flagged one other thing for me, which is potentially worth conversation:

JUCE 8 changed the open-source license from GPLv3 → AGPLv3. The commercial license terms also changed. This may warrant review depending on how downstream users of this template consume it.