Release 4.5.3 - SoftwareSerial and SPI fixes

Overview

Important update if you're a SoftwareSerial or SerialPIO user for much improved reception performance. Otherwise, just minor bug fixes

SoftwareSerial/SerialPIO

• Optimize parity calculations in SerialPIO (#2932) (#2933)
• Rewrite SerialPIO receive path, ensure proper edge (#2929)

Peripherals

• Fix RP2350B SPI RX pin list (#2931)
• Fix rp2040.getCycleCount() from core1 (#2915)
• Add pin definitions for I2C1 on Cytron IRIV (#2916)
• Generate PWMAudio pacer frequencies for 176/276MHz (#2913)

Miscellaneous

• Add PIO.h header verification to CI (#2911)
• Update LittleFS and Adafruit USB libraries (#2919)