Use 3.1.x for C# in CI #995

Job	Run time
C++ Clang 9 (Ubuntu 20.04)	1m 30s
C++ Clang 10 (Ubuntu 20.04)	1m 31s
C++ Clang 13 (Ubuntu 22.04)	1m 26s
C++ GCC 6 (Ubuntu 20.04)	1m 19s
C++ Clang 11 (Ubuntu 20.04)	1m 28s
C++ Clang 14 (Ubuntu 22.04)	1m 53s
C++ GCC 11 (Ubuntu 22.04)	1m 15s
C++ GCC 7 (Ubuntu 20.04)	1m 30s
C++ Clang 12 (Ubuntu 20.04)	1m 35s
C++ Clang 15 (Ubuntu 22.04)	1m 24s
C++ GCC 12 (Ubuntu 22.04)	1m 34s
C++ GCC 8 (Ubuntu 20.04)	1m 31s
C++ Clang 16 (Ubuntu 22.04)	1m 21s
C++ GCC 13 (Ubuntu 22.04)	1m 28s
C++ Clang 17 (Ubuntu 22.04)	1m 18s
C++ GCC 9 (Ubuntu 20.04)	1m 22s
C# 3.1.x	3m 15s
C++ GCC 10 (Ubuntu 20.04)	1m 40s
Java 8 (ubuntu-22.04)	1m 12s
Java 8 (windows-latest)	2m 4s
C++ Xcode (macOS)	1m 29s
C++ MSVC (Windows) (windows-latest)	4m 35s
Java 17 (ubuntu-22.04)	1m 36s
Java 17 (windows-latest)	2m 41s
Rust stable	44s
Java 21 (ubuntu-22.04)	1m 34s
Rust beta	56s
Java 21 (windows-latest)	2m 8s
Rust nightly	46s
	48m 5s

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