Bump actions/checkout from 3 to 4 #271

Job	Run time
Static analysis	19s
Run static analysis on the tests themselves	16s
Code style	15s
Test on 8.2 - stable deps	3m 49s
Test on 8.1 - high deps	3m 34s
Test on 8.2 - high deps	3m 43s
Test on 8.1 - stable deps	3m 42s
Test on 8.3 - stable deps	3m 44s
Test on 8.2 - low deps	3m 25s
Test on 8.4 - stable deps	6m 42s
Test on 8.4 - high deps	6m 47s
Test on 8.3 - high deps	4m 1s
Test on 8.1 - low deps	3m 23s
Test on 8.3 - low deps	3m 46s
	47m 26s

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