bump go to 1.20; drop pkg/errors #13

Job	Run time
Build and Test (1.15, ubuntu-latest)	15s
Build and Test (1.15, windows-latest)	58s
Build and Test (1.15, macos-latest)	56s
Build and Test (1.16, ubuntu-latest)	22s
Build and Test (1.16, windows-latest)	59s
Build and Test (1.16, macos-latest)	25s
Build and Test (1.17, ubuntu-latest)	17s
Build and Test (1.17, windows-latest)	1m 0s
Build and Test (1.17, macos-latest)	53s
Build and Test (1.18, ubuntu-latest)	16s
Build and Test (1.18, windows-latest)	50s
Build and Test (1.18, macos-latest)	54s
Build and Test (1.19, ubuntu-latest)	12s
Build and Test (1.19, windows-latest)	46s
Build and Test (1.19, macos-latest)	23s
Build and Test (1.20, ubuntu-latest)	32s
Build and Test (1.20, windows-latest)	59s
Build and Test (1.20, macos-latest)	24s
Build and Test (1.21, ubuntu-latest)	41s
Build and Test (1.21, windows-latest)	57s
Build and Test (1.21, macos-latest)	1s
	13m 0s

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