Use LittleFS instead of SPIFFS

[joelkoz]

SPIFFS has been deprecated and generates a lot of deprecation warnings when compiling. LittleFS is the preferred flash file system according to expressif documentation. LittleFS is API compatible, but is not implementation compatible. Switching to LittleFS would require pre-existing devices to erase any saved configuration and start over.

https://arduino-esp8266.readthedocs.io/en/latest/filesystem.html#spiffs-deprecation-warning