Bump to 300.2.3 to address riscv64 support #446

Job	Run time
Rustfmt	13s
Test (x86_64-unknown-linux-gnu, stable, ubuntu-latest)	4m 48s
Test (x86_64-unknown-linux-gnu, beta, ubuntu-latest)	4m 48s
Test (x86_64-unknown-linux-gnu, nightly, ubuntu-latest)	4m 53s
Test (x86_64-apple-darwin, stable, macos-latest)	4m 55s
Test (aarch64-apple-darwin, nightly, macos-latest)	4m 22s
Test (i686-unknown-linux-gnu, stable, ubuntu-latest)	2m 51s
Test (arm-unknown-linux-gnueabi, stable, ubuntu-latest)	3m 0s
Test (aarch64-unknown-linux-gnu, stable, ubuntu-latest)	3m 7s
Test (riscv64gc-unknown-linux-gnu, stable, ubuntu-latest)	2m 52s
Test (x86_64-unknown-linux-musl, stable, ubuntu-latest)	2m 29s
Test (x86_64-pc-windows-gnu, stable, ubuntu-latest)	5m 19s
Test (i686-pc-windows-msvc, stable-i686-msvc, windows-2019)	18m 57s
Test (i686-pc-windows-msvc, stable-i686-msvc, windows-2019, yes)	20m 24s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest)	19m 49s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest, yes)	19m 57s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest, installed)	20m 43s
	2h 23m 27s

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