fix(boards): Disable uart serial node in Xiao BLE by default

[caksoylar]

xiao_serial node corresponds to uart0, which uses D6 and D7 pins by default on Xiaos. When logging or Studio USB snippets are used, they turn on CONFIG_SERIAL and this prevents those pins from being used for other things, even though both use serial over USB.

For RP2040 this is disabled in https://github.com/zmkfirmware/zmk/blob/main/app/boards/seeeduino_xiao_rp2040.overlay#L7, for Hummingbird shield this was disabled at the shield level but was (mistakenly?) removed in e0339a2#diff-b0c154eb8d05802c1c61671a7e9e2ca6021c2ec81127f2c57194a84d7a85cdbfL57.

Is there a reason to not disable it by default for ZMK at the board level, like this? I can't think of any. Other boards like nice!nano don't set status = "okay"; either.

Fixes #1887.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.

[petejohanson]

This should be fine. AFAIK, no one is using the serial port on these for anything, and when they do, we can check then if the overlay ordering causes any issues. Thanks!