feat: remove .NET Core 3.1 from skwas.MockHttp.Server (EOL) #104

Job	Run time
determine-version	18s
determine-version	13s
determine-version	8s
build (v${{ needs.determine-version.outputs.version }})	0s
build (v4.4.1-pr122.23)	1m 31s
build (v4.4.1-pr122.23)	1m 31s
pack	0s
pack	50s
pack	1m 6s
test	0s
deploy	0s
deploy	0s
deploy	0s
test (ubuntu-latest)	2m 50s
test (ubuntu-latest)	2m 36s
test (windows-latest)	4m 15s
test (windows-latest)	4m 1s
test (macos-13)	3m 58s
test (macos-13)	4m 32s
	27m 49s

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