ORC-1854: Remove `ubuntu20` from `os-list.txt` #4518

Job	Run time
Docker debian11	21m 50s
Docker debian12	21m 10s
Docker ubuntu24	20m 14s
Docker fedora37	20m 26s
Docker oraclelinux9	20m 17s
Java 17 and clang++ on ubuntu-22.04	20m 17s
Java 21 and clang++ on ubuntu-22.04	20m 47s
cpp-linter	8m 43s
Java 17 and clang++ on ubuntu-24.04	20m 27s
Java 21 and clang++ on ubuntu-24.04	19m 56s
Java 17 and clang++ on macos-13	21m 52s
Javadoc generation	1m 32s
Java 21 and clang++ on macos-13	21m 23s
Java 17 and clang++ on macos-14	14m 7s
License Check	33s
Java 21 and clang++ on macos-14	16m 17s
Java 17 and clang++ on macos-15	13m 53s
Java 21 and clang++ on macos-15	13m 46s
C++ Test on macOS (13)	7m 41s
Java 17 and g++ on ubuntu-22.04	20m 26s
C++ Test on macOS (14)	4m 4s
SIMD programming using C++ intrinsic functions on ubuntu-22.04	9m 37s
C++ General Test on Windows	5m 40s
C++ AVX512 Test on Windows	6m 13s
	5h 51m 11s

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