merge 1.6.0 into master

[bxparks]

• 1.6.0 (2023-06-08)
  • Add overloaded flash string versions of copyReplaceChar() and
    copyReplaceString() which accept __FlashStringHelper* pointers in the
    source string parameters.
  • Support keywords[] in PROGMEM in KString class.
  • Update supported boards and tiers
    • Add SAMD21 and SAMD51 boards to Tier 1
      • Add 2 SAMD boards from 2 different companies, to test their
        Arduino Core software:
        • Seeeduino XIAO M0 (SAMD21 48MHz ARM Cortex-M0+)
        • Adafruit ItsyBitsy M4 (SAMD51 120MHz ARM Cortex-M4)
      • SAMD21 and SAMD51 boards are back in Tier 1, as long as they use
        the traditional Arduino API instead of the new
        Arduino-Core.
      • Fortunately most third party SAMD21 and SAMD51 boards continue to
        use the traditional Arduino API.
    • Move Teensy 3.2 to Tier 2
      • This board is entering end-of-life.
      • As well, the Teensyduino environment integrates with the Arduino
        IDE and CLI in a way that's different than all other third-party
        Arduino boards. Some of my automation scripts do not work with
        Teensyduino, so it becomes very time consuming to test the Teensy
        boards.
      • All Teensy boards are now in Tier 2 ("Should work but not tested
        often").