Bump version to 2.60.0-dev #4140

Job	Run time
Run release tests	22s
Prepare supported CodeQL environment matrix	5s
Run Guideline Recategorization tests	16s
Run analysis report tests (2.21.4, codeql-cli/v2.21.4, codeql-bundle-v2.21.4)	5m 55s
	6m 38s

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