Bump the minimum supported version of CodeQL to 2.15.5 #3432

Job	Run time
Diagnostic export (ubuntu-latest, linked)	1m 2s
Diagnostic export (macos-latest, linked)	1m 49s
Diagnostic export (windows-latest, linked)	3m 10s
Diagnostic export (ubuntu-latest, nightly-latest)	55s
Diagnostic export (macos-latest, nightly-latest)	1m 24s
Diagnostic export (windows-latest, nightly-latest)	3m 52s
	12m 12s

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