Bump actions/checkout from 5 to 6 (#2392) #1507

Job	Run time
Windows - PHP 8.1 & SQL Server	2m 30s
testsuite (8.1, mysql)	1m 9s
testsuite (8.4, mysql)	1m 2s
testsuite (8.1, sqlite)	31s
testsuite (8.4, pgsql)	54s
testsuite (8.1, pgsql)	38s
testsuite (8.1, sqlite, prefer-lowest)	29s
Coding Standard & Static Analysis	43s
testsuite (8.2, mysql)	1m 1s
testsuite (8.4, sqlite)	32s
	9m 29s

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