remove ubuntu-20 #1143

Job	Run time
Test (ubuntu-22.04, stable)	15s
Test (ubuntu-22.04, beta)	14s
Test (ubuntu-22.04, nightly)	14s
Test (ubuntu-24.04, stable)	21s
Docker (aarch64-unknown-linux-gnu)	48s
iOS (aarch64-apple-ios, iphoneos)	16s
Docker (arm-unknown-linux-gnueabihf)	41s
Test (ubuntu-24.04, beta)	17s
iOS (x86_64-apple-ios, iphonesimulator)	15s
Docker (armv7-unknown-linux-gnueabihf)	38s
Test (ubuntu-24.04, nightly)	17s
Docker (i586-unknown-linux-gnu)	37s
Test (macos-latest, stable)	11s
Docker (i686-unknown-linux-gnu)	42s
Windows AArch64	1m 23s
Test (macos-latest, nightly)	14s
Docker (powerpc64-unknown-linux-gnu)	46s
Test (windows-latest, stable-x86_64-msvc)	1m 27s
Docker (s390x-unknown-linux-gnu)	37s
Test (windows-latest, stable-i686-msvc)	24s
Build Targets (wasm32-unknown-unknown)	14s
Docker (x86_64-pc-windows-gnu)	46s
Build Targets (wasm32-wasi)	9s
Test (windows-latest, stable-x86_64-gnu)	40s
Docker (x86_64-unknown-linux-gnu)	34s
Build Targets (x86_64-unknown-fuchsia)	15s
Test (windows-latest, nightly-x86_64-msvc)	29s
Docker (x86_64-unknown-linux-musl)	27s
Test (windows-latest, nightly-i686-msvc)	30s
Build Targets (x86_64-fortanix-unknown-sgx)	15s
Docker (arm-linux-androideabi)	1m 33s
Test (windows-latest, nightly-x86_64-gnu)	34s
Build Targets (x86_64-unknown-illumos)	15s
Docker (armv7-linux-androideabi)	1m 35s
Docker (aarch64-linux-android)	1m 29s
Docker (i686-linux-android)	1m 29s
Miri	2m 41s
Rustfmt	4s
Docker (x86_64-linux-android)	1m 19s
MSRV (ubuntu-22.04)	14s
MSRV (windows-latest)	31s
	26m 40s

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