Mark board_M5StampPLC deprecated

[ainyan03]

Mark board_M5StampPLC deprecated

• The product is named StamPLC; board_M5StamPLC was added in Add board_M5StamPLC as the correctly spelled name of board_M5StampPLC #309 and the old spelling was kept as a plain alias. The old spelling now carries a deprecation warning pointing to board_M5StamPLC.
• The value is unchanged, so a board identity stored in NVS still resolves.
• M5Unified switches to the new name in Use board_M5StamPLC instead of the old spelling board_M5StampPLC M5Unified#408; merge that first so M5Unified builds stay free of the warning.

Testing

• Local build matrix of this repository's and M5Unified's CI versions (Arduino 2.x / 3.x, ESP-IDF 5.1 to 6.1, all chips) plus the native (SDL) build, with Use board_M5StamPLC instead of the old spelling board_M5StampPLC M5Unified#408: all passed with no deprecation warning from the libraries. A one-line check confirms the warning on the old name and none on the new name. CI on the fork: passed.