remove ubuntu-20 #1160

Job	Run time
Miri	2m 50s
iOS (aarch64-apple-ios, iphoneos)	13s
Docker (aarch64-unknown-linux-gnu)	34s
iOS (x86_64-apple-ios, iphonesimulator)	16s
Build Targets (wasm32-unknown-unknown)	14s
Docker (arm-unknown-linux-gnueabihf)	33s
MSRV (ubuntu-24.04)	15s
Build Targets (wasm32-wasip1)	16s
Docker (armv7-unknown-linux-gnueabihf)	36s
MSRV (windows-latest)	44s
Build Targets (x86_64-unknown-fuchsia)	17s
Docker (i586-unknown-linux-gnu)	37s
Rustfmt	4s
Build Targets (x86_64-fortanix-unknown-sgx)	17s
Docker (i686-unknown-linux-gnu)	34s
Build Targets (x86_64-unknown-illumos)	15s
Docker (powerpc64-unknown-linux-gnu)	44s
Test (ubuntu-24.04, stable)	1m 56s
Docker (s390x-unknown-linux-gnu)	37s
Windows AArch64	36s
Test (ubuntu-24.04, beta)	2m 6s
Docker (x86_64-pc-windows-gnu)	52s
Test (ubuntu-24.04, nightly)	2m 17s
Docker (x86_64-unknown-linux-gnu)	26s
Test (macos-latest, stable)	1m 1s
Docker (x86_64-unknown-linux-musl)	27s
Test (macos-latest, nightly)	1m 18s
Docker (arm-linux-androideabi)	1m 13s
Test (windows-latest, stable-x86_64-msvc)	2m 13s
Docker (armv7-linux-androideabi)	1m 23s
Test (windows-latest, stable-i686-msvc)	2m 9s
Docker (aarch64-linux-android)	1m 36s
Test (windows-latest, stable-x86_64-gnu)	2m 9s
Docker (i686-linux-android)	1m 34s
Test (windows-latest, nightly-x86_64-msvc)	1m 45s
Docker (x86_64-linux-android)	1m 31s
Test (windows-latest, nightly-i686-msvc)	2m 17s
Test (windows-latest, nightly-x86_64-gnu)	2m 32s
	41m 17s

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