Bump version to 2.60.0-dev #4720

Job	Run time
Prepare CodeQL unit test matrix	7s
Run unit tests (c, 2.21.4, codeql-cli/v2.21.4, codeql-bundle-v2.21.4, ubuntu-latest-xl, codeql-cl...	47m 2s
Run unit tests (cpp, 2.21.4, codeql-cli/v2.21.4, codeql-bundle-v2.21.4, ubuntu-latest-xl, codeql-...	2h 16m 50s
Validate test results	8s
	3h 4m 7s

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