Bump version to 2.50.0-dev #3188

Job	Run time
Prepare supported CodeQL environment matrix	6s
Run Guideline Recategorization tests	13s
Run release tests	15s
Run analysis report tests (2.20.7, codeql-cli/v2.20.7, codeql-bundle-v2.20.7)	4m 50s
	5m 24s

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